LUBRICATING OIL COMPOSITION

Abstract

A lubricating oil composition comprising a zinc dithiophosphate and a zinc saturated aliphatic monocarboxylate dissolved in a base oil, wherein the zinc dithiophosphate and zinc saturated aliphatic monocarboxylate are in the ratio of equivalents from 6:1 to 45:1.

Description

[0001] The present invention relates to a lubricating oil composition, and particularly relates to a lubricating oil composition favorably employable for hydraulic systems and power transmission devices.

BACKGROUND OF THE INVENTION

[0002] Lubricating oils are generally prepared by adding various additives to a base oil. Examples of base oils include mineral oil, synthetic oil, and natural oil, such as vegetable oil and its modified products.

[0003] Zinc dithiophosphate, such as zinc dialkyldithiophosphate or zinc dialkylaryldithiophosphate, is employed in hydraulic oil for power transmission systems of automobiles and hydraulic systems as a requisite additive because it imparts anti-oxidation property and anti-abrasion property to lubricating oils.

[0004] Recently, most of lubricating oils, such as hydraulic oils, are used under increased severe conditions, for instance, at elevated temperatures and for a prolonged time. The zinc dithiophosphate is stable under normal conditions but is liable to decompose at high temperatures or after the use for a long period. The zinc dithiophosphate produces oil-insoluble precipitates by its decomposition. The oil-insoluble precipitates are converted into sludge. When the lubricating oil is a hydraulic oil, the produced sludge causes plugging of filters of the hydraulic system and disturbs normal operation of the hydraulic system. Moreover, the decomposition of zinc dithiophosphate causes lowering of oxidation stability and abrasion resistance of the lubricating oil. Accordingly, the decomposition of zinc dithiophosphate results in lower durability of the lubricating oil.

[0005] For this reason, it is desired to have a lubricating oil composition having high durability, that is, showing high thermal stability and high anti-oxidation property, and producing little sludge when it is used under severe conditions, such as at high temperatures, for a long time, and particularly at a high temperature for a long time.

[0006] U.S. Pat. No. 4,263,150 teaches that treatment of mixed metal salts of dialkyl phosphorodithioic and carboxylic acids with secondary or tertiary phosphites materially decreases copper corrosion. The ratio of equivalents of phosphorus acid to carboxylic acid may be from about 0.1:1 to about 30:1, preferably from about 0.5:1 to about 20:1. The examples show a ratio of equivalents of 4:1.

[0007] U.S. Pat. No. 4,308,154 teaches the use of certain metal carboxylate salts to improve the thermal stability of lubricating compositions and functional fluids containing metal dithiophosphate salts. The ratio of equivalents of dialkylphosphorodithioic acids to carboxylic acids is between 2.5:1 and 4.5:1.

[0008] U.S. Pat. No. 4,417,990 teaches an additive combination comprising metal salts of certain dithiophosphates and carboxylates and sulfurized Group 11 metal phenates. The ratio of equivalents of dialkylphosphorodithioic acids to carboxylic acids is between 0.5:1 and 400:1, but all examples have a ratio of 4:1.

[0009] U.S. Pat. No. 4,466,894 teaches an additive combination comprising metal salts of optionally substituted phosphorodithioic acid and C1-C29 aliphatic or alicyclic carboxylic acid; a sulfurized Group II metal phenate; and an optionally substituted benzotriazole. The ratio of equivalents of dialkylphosphorodithioic acids to carboxylic acids is between 0.5:1 and 500:1, preferably 0.5:1 and 20:1, but all examples have a ratio of 4:1.

[0010] WO Publication No. 84/04322 teaches an additive combination comprising metal salts of phosphorodithioic acid and C2-C40 aliphatic or alicyclic carboxylic acid; a sulfurized Group II metal phenate; and a triazole. The ratio of equivalents of dialkylphosphorodithioic acids to carboxylic acids is between 0.5:1 and 500:1, preferably 0.5:1 and 20:1, but all examples have a ratio of 4:1.

SUMMARY OF THE INVENTION

[0011] The present invention is based in part on the discovery that the use of zinc saturated aliphatic monocarboxylate in combination with zinc dithiophosphate, when the ratio of equivalents of dithiophosphate to monocarboxylate is from 6:1 to 45:1, can improve sludge forming tendency of a lubricating oil composition at high temperature conditions. Preferably, the ratio of equivalents is from 7:1 to 12:1.

[0012] Preferably, the total amount of both zinc salts is in the range of 50 to 10,000 ppm expressed in terms of their zinc content, the base oil is a mineral oil having a viscosity in the range of 2 to 500 cSt at 40° C., the zinc dithiophosphate is zinc di-2-ethylhexyldithiophosphate, and the zinc saturated aliphatic monocarboxylate is zinc 2-ethylhexanoate.

[0013] In one embodiment, the zinc dithiophosphate and zinc saturated aliphatic monocarboxylate are in the ratio of 1:0.001 to 1:0.08 by weight, and the total amount of both zinc salts is in the range of 50 to 10,000 ppm expressed in terms of their zinc content. Preferably, the zinc dithiophosphate and zinc saturated aliphatic monocarboxylate are in the ratio of 1:0.008 to 1:0.07 by weight, and the total amount of both zinc salts is in the range of 100 to 1,000 ppm expressed in terms of their zinc content.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to assist the understanding of this invention, reference will now be made to the appended drawings. The drawings are exemplary only, and should not be construed as limiting the invention.

[0015] The FIGURE shows a plot of sludge precipitation period (in hours) for different ratios of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate.

DETAILED DESCRIPTION OF THE INVENTION

[0016] In its broadest aspect, the present invention involves a lubricating oil composition having a zinc dithiophosphate and a zinc saturated aliphatic monocarboxylate dissolved in a base oil. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is from 6:1 to 45:1, more preferably from 7:1 to 12:1.

[0017] Preferably, the total amount of both zinc salts is in the range of 50 to 10,000 ppm expressed in terms of their zinc content, more preferably from 100 to 1,000 ppm

The Base Oil

[0018] The lubricating oil composition of the invention is prepared by adding the additives to a base oil in the known manner. The base oil is selected from the known oils for lubricating oils from the viewpoint of the purpose of its use, the required performance, and so forth. Therefore, mineral oil is generally used as the base oil. However, if desired, synthetic oils such as synthetic hydrocarbon, fatty acid esters, and phosphoric acid ester or vegetable oils such as fatty acid glycerides can be employed. If the lubricating oil composition of the invention is prepared for the use as a hydraulic oil, the base oil preferably has a viscosity of 2 to 500 cSt at 40° C.

Zinc Dithiophosphate

[0019] The zinc dithiophosphate can be zinc dialkyldithiophosphate or zinc dialkylaryldithiophosphate. Their derivatives also can be employed. Generally employed zinc dialkyldithiophosphate has an alkyl group of 3 to 18 carbon atoms. Preferred alkyl group contains 4 to 12 carbon atoms, and most preferred alkyl group contains 6 to 10 carbon atoms. Generally employed zinc dialkylaryidithiophosphate has an alkyl phenyl group having alkyl of 6 to 15 carbon atoms. These zinc dithiophosphates can be employed singly or in combination. Preferred is the single use of zinc dithiophosphate having branched alkyl chain, for instance, zinc di-2-ethylhexyldithiophosphate. Also preferred is a mixture of zinc dithiophosphates containing at least 50 wt. % (especially, at least 70 wt. %) of the zinc salt of the branched alkyl-type dithiophosphate acid.

[0020] The zinc dithiophosphate is generally produced and supplied in the form of a solution containing it in mineral oil at a high concentration. Accordingly, the lubricating oil composition of the invention is generally prepared using such concentrated solution of zinc dithiophosphate. However, the amount of zinc dithiophosphate described in the specification is calculated on the solid content basis.

Zinc Saturated Aliphatic Monocarboxylate

[0021] The zinc saturated aliphatic monocarboxylate, that is, zinc salt of a couple of a saturated aliphatic monocarboxylic acid, can be zinc salt (normal salt) of a saturated aliphatic monocarboxylic acid having 4 to 18 carbon atoms. Preferred is a zinc salt of a couple of saturated aliphatic monocarboxylic acid having 4 to 12 (particularly 6 to 10). The zinc carboxylate can be used singly or in combination. Particularly preferred is the single use of zinc saturated aliphatic monocarboxylate having branched alkyl chain, for instance, zinc di-2-ethylhexanoate (herein referred to simply as “zinc di-2-ethylhexanoate”). Also preferred is a mixture of zinc saturated aliphatic monocarboxylate containing at least 50 wt. % (especially, at least 70 wt. %) of the zinc salt of the branched alkyl-type saturated aliphatic monocarboxylic acid.

[0022] The zinc saturated aliphatic monocarboxylate can be produced and supplied in the form of a solution containing it in mineral oil at a high concentration. Accordingly, the lubricating oil composition of the invention can be prepared using such concentrated solution of zinc saturated aliphatic monocarboxylate. However, the amount of zinc saturated aliphatic monocarboxylate described in the specification is calculated on the solid content basis.

Zinc Dithiophosphate and Zinc Saturated Aliphatic Monocarboxylate Mixture

[0023] The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is from 6:1 to 45:1, more preferably from 7:1 to 12:1.

[0024] When the zinc dithiophosphate is zinc di-2-ethylhexyldithiophosphate and the zinc saturated aliphatic monocarboxylate is zinc 2-ethylhexanoate, the zinc dithiophosphate and zinc saturated aliphatic monocarboxylate are contained in a weight ratio of 1:0.001 to 1:0.08 (former:latter), and the total amount of both zinc salts is in the range of 50 to 10,000 ppm expressed in terms of their zinc content. If the weight ratio is outside the above range, or if both salts are used to give a total zinc content outside the range of 50 to 10,000 ppm, it is difficult to attain the object of the invention, that is, reduction of deterioration of the lubricating oil in the use at high temperatures and/or for long period of time. Preferred weight ratio between zinc dithiophosphate and zinc saturated aliphatic monocarboxylate is in the range of 1:0.008 to 1:0.07 (former:latter), and preferred total zinc content of both zinc salts is in the range of 100 to 1,000 ppm.

Other Additives

[0025] The lubricating oil composition of the invention may further contain other additives such as a detergent-dispersant such as a metal salt of sulfonate, an oxidation inhibitor other than the zinc dithiophosphate, a corrosion inhibitor, and an anti-emulsifier. If desired, a pour point depressant such as polymethyl methacrylate can be added.

[0026] As the additives for lubricating oils, such as the detergent-dispersants, oxidation inhibitors, rust inhibitors, anti-emulsifiers, and pour point depressants, a variety of additives are known. These known additives or their analogous compounds can be used, if desired, when the lubricating oil composition of the invention is prepared. Representative examples of the additives are described below.

[0027] A metal phenate or a metal sulfonate is generally used as the detergent-dispersant. The metal phenate is an alkaline earth metal salt of alkylphenol sulfide having an alkyl group of approximately 8 to 30 carbon atoms. Generally employed alkaline earth metals are calcium, magnesium and barium. Preferably the metal sulfonate is an alkaline earth metal salt of a sulfonated aromatic compound or a sulfonated mineral oil having a molecular weight of approximately 400 to 600. Generally employed alkaline earth metals are also calcium, magnesium and barium. The metal phenate and metal sulfonate can be used singly or in combination. Also employable are other metal-containing detergents such as salicylates, phosphorates and naphthenates of alkaline earth metals. These detergent-dispersants can be employed singly or in combination. The aforementioned phenate and sulfonate can be employed in combination with these other metal-containing detergents. The metal-containing detergents can be of a neutral type or of an over-based type having an alkalinity value of 150 to 300 or more. If desired, an ashless type dispersant (which may contain boron) can be used singly or in combination.

[0028] The detergent-dispersant is generally used in the lubricating oil composition at a concentration of 0.01 to 10 wt. %.

[0029] Examples of oxidation inhibitors other than zinc dithiophosphate include various known oxidation inhibitors such as additives of phenol-type, amine-type, phosphor-containing type and sulfur-containing type. These oxidation inhibitors can be optionally used.

[0030] The pour point depressant generally is polyalkyl methacrylate.

[0031] The rust inhibitor generally is alkenylsuccinic acid, its salt, ester, or amine derivative.

[0032] The anti-emulsifier generally is polyalkylene glycol.

[0033] The lubricating oil composition of the invention can further contain various auxiliary additives other than those mentioned above. Examples of the auxiliary additives include know extreme pressure agents, corrosion inhibitors, friction modifiers, and antifoaming agents. An antiabrasion improver and a multi-functional additive (e.g., an organic molybdenum compound such as molybdenum dithiophosphate) can be used in combination.

[0034] The additives can be added simultaneously or separately to a base oil when the lubricating oil composition of the invention is prepared. Otherwise, the zinc dithiophosphate and zinc saturated aliphatic monocarboxylate can be added to one portion of a base oil at a high concentration, and then thus obtained concentrated solution can be diluted with another portion of the base oil to prepare the lubricating oil composition.

EXAMPLES

[0035] The invention will be further illustrated by following examples, which set forth particularly advantageous method embodiments. While the Examples are provided to illustrate the present invention, they are not intended to limit it.

Example 1

[0036] To 99.596 g of a base oil (mineral oil having a viscosity index of 98 and showing a viscosity of 32 cSt at 40° C.) were added 0.40 g of zinc di-2-ethylhexyldithiophosphate and 0.004 g of zinc 2-ethylhexanoate to give a lubricating oil composition containing di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio of 1:0.01 and having a zinc content of 365 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 45.6:1.

Example 2

[0037] The procedure of Example 1 was repeated except that 99.585 g of the base oil was used and the amount of zinc 2-ethylhexyldithioate was changed to 0.015 g to give a lubricating oil composition containing di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.0375 and having a zinc content of 389 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 12.2:1.

Example 3

[0038] The procedure of Example 1 was repeated except that 99.574 g of the base oil was used and the amount of zinc 2-ethylhexyldithioate was changed to 0.026 g to give a lubricating oil composition containing di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.065 and having a zinc content of 389 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 7.0:1.

Example 4

[0039] The procedure of Example 1 was repeated except that 99.57 g of the base oil was used and the amount of zinc 2-ethylhexyldithioate was changed to 0.03 g to give a lubricating oil composition containing di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.075 and having a zinc content of 422 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 6.1:1.

Comparison Example A

[0040] The procedure of Example 1 was repeated except that 99.60 g of the base oil was used and zinc 2-ethylhexanoate was not used to give a lubricating oil composition containing di-2-ethylhexyldithiophosphate only and having a zinc content of 356 ppm.

Comparison Example B

[0041] The procedure of Example 1 was repeated except that 99.559 g of the base oil was used and the amount of zinc 2-ethylhexanoate was changed to 0.041 g to give a lubricating oil composition containing di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.103 and having a zinc content of 446 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 4.5:1.

Comparison Example C

[0042] The procedure of Example 1 was repeated except that 99.555 g of the base oil was used and the amount of zinc 2-ethylhexanoate was changed to 0.045 g to give a lubricating oil composition containing di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.113 and having a zinc content of 445 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 4.05:1.

Comparison Example D

[0043] The procedure of Example 1 was repeated except that 99.435 g of the base oil was used and the amount of zinc 2-ethylhexanoate was changed to 0.165 g to give a lubricating oil composition containing di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.41 and having a zinc content of 719 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 1.11:1.

Thermal Stability Test

[0044] (1) Test Method

[0045] In a 50 ml-beaker was placed 40 ml of a lubricating oil composition sample. The beaker was then placed in a forced-air circulation oven controlled at 160° C.±1° C. The oil composition sample was taken out every one hour to check precipitation of sludge and determine the time when sludge began to precipitate. This test means that thermal stability of a lubricating oil composition is higher in the case that the period is longer.

[0046] (2) Test Results

	Examples	Com. Examples
	1	2	3	4	A	B	C	D
Equivalent	45.6	12.2	7.0	6.1	—	4.5	4.05	1.11
Ratio
Sludge	88	96	90	86	46	56	55	48
Precipitation
Period (hours)

[0047] The above results indicate that the lubricating oil compositions of the invention containing zinc dithiophosphate and zinc carboxylate in the specific ratio shows thermal stability higher than the lubricating oil composition containing only zinc dithiophosphate or containing the zinc carboxylate at a large amount.

[0048] Note that the sludge formation time is at least 53% better when the ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is from 6:1 to 45:1, and that best results are obtained with ratios of from 7:1 to 12:1.

Example 5

[0049] To 99.216 g of a base oil (mineral oil having a viscosity index of 98 showing a viscosity of 32 cSt at 40° C.) were added 0.40 g of zinc di-2-ethylhexyldithiophosphate and 0.004 g of zinc 2-ethylhexanoate and further 0.10 g of 2, 6-di-tert-p-cresol, 0.20 g of basic calcium petroleum-sulfonate, 0.07 g of tetrapropenylsuccinic acid, and 0.01 g of polyoxyalkylene anti-emulsifier, to give a lubricating oil composition containing di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio of 1:0.01 and having a zinc content of 365 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 45.6:1.

Example 6

[0050] The procedure of Example 5 was repeated except that 99.205 g of the base oil was used and the amount of zinc 2-ethylhexanoate was changed to 0.015 g to give a lubricating oil composition containing zinc di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.0375 and having a zinc content of 389 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 12.

Example 7

[0051] The procedure of Example 5 was repeated except that 99.194 g of the base oil was used and the amount of zinc 2-ethylhexyldithioate was changed to 0.026 g to give a lubricating oil composition containing zinc di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.065 and having a zinc content of 413 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 7.0:1.

Example 8

[0052] The procedure of Example 5 was repeated except that 99.19 g of the base oil was used and the amount of zinc 2-ethylhexyldithioate was changed to 0.03 g to give a lubricating oil composition containing zinc di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.075 and having a zinc content of 422 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 6.1:1.

Comparison Example E

[0053] The procedure of Example 5 was repeated except that 99.22 g of the base oil was used and zinc 2-ethylhexanoate was not used to give a lubricating oil having a zinc content of 413 ppm.

Comparison Example F

[0054] The procedure of Example 5 was repeated except that 99.179 g of the base oil was used and the amount of zinc 2-ethylhexanoate was changed to 0.165 g to give a lubricating oil composition containing zinc di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.103 and having a zinc content of 719 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 4.5:1.

Comparison Example G

[0055] The procedure of Example 5 was repeated except that 99.055 g of the base oil was used and the amount of zinc 2-ethylhexanoate was changed to 0.165 g to give a lubricating oil composition containing zinc di-2-ethylhexyldithiophosphate and zinc 2-ethylhexanoate at a weight ratio 1:0.41 and having a zinc content of 719 ppm. The ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is about 1.11:1.

Evaluation Test as Hydraulic Oil

[0056] (1) Thermal Stability Test ASTM-D-2070 (Cincinnati Millacron Thermal Stability A Method) In this test, less production of sludge (mg of sludge production per 100 ml of test oil) means a lubricating oil of higher thermal stability.

[0057] (2) Oxidation Stability Test

[0058] JIS-K-2514 (according to this rotary cylinder type oxidation stability test method:150° C.)

[0059] In this test, a longer life time before oxidation means a hydraulic oil having higher oxidation stability.

[0060] (3) Abortion Resistance Test

[0061] JIS-K-2514 (according to the oxidation stability test for internal engine lubricating oil: test oil was deteriorated by keeping it at 150° C. for 24 hours, and the deteriorated oil was placed in a shell four ball type friction abrasion test machine and subjected to friction-abrasion at a pressure of 30 kgf, a revolution of 1,800 r.p.m. for 30 min.)

[0062] In this test, a smaller wear track diameter means a lubricating oil having a higher friction resistance.

[0063] (4) Test Results

	Examples	Com. Examples
	5	6	7	8	E	F	G
Equivalent Ratio	45.6	12.2	7.0	6.1	—	4.5	1.11
Thermal Stability Test	3	1	2	5	153	23	95
Amount of Sludge (mg)
Oxidation Stability	355	341	343	340	352	321	265
Test Life Time (min.)
Abrasion Resistance
Test Wear Track
Diameter (mm)	0.52	0.56	0.60	0.60	1.20	0.64	0.66

[0064] The above results indicate that the lubricating oil compositions of the invention containing zinc dithiophosphate and zinc carboxylate in the specific ratio shows better balanced characteristics in thermal stability, oxidation stability and abrasion resistance, than the lubricating oil composition containing only zinc dithiophosphate or contain the zinc carboxylate at a large amount.

[0065] The lubricating oil composition of the invention is high in high temperature stability and oxidation stability and produces less sludge in its use at high temperatures or for a long period of time, particularly under sever conditions such as a combination of high temperatures and prolonged use. Accordingly, the lubricating oil composition of the invention has excellent durability, and is advantageously employable for lubricating hydraulic systems and power transmission devices.

[0066] While the present invention has been described with reference to specific embodiments, this application is intended to cover those various changes and substitutions that may be made by those skilled in the art without departing from the spirit and scope of the appended claims.

Claims

1. A lubricating oil composition comprising a zinc dithiophosphate and a zinc saturated aliphatic monocarboxylate dissolved in a base oil, wherein the ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is from 6:1 to 45:1.

2. A lubricating oil composition according to claim 1 wherein the ratio of equivalents of zinc dithiophosphate to zinc saturated aliphatic monocarboxylate is from 7:1 to 12:1.

3. A lubricating oil composition according to claim 1 wherein the total amount of both zinc salts is in the range of 50 to 10,000 ppm expressed in terms of their zinc content.

4. The lubricating oil composition of claim 1, wherein the base oil is a mineral oil having a viscosity in the range of 2 to 500 cSt at 40° C.

5. The lubricating oil composition of claim 1, wherein the zinc dithiophosphate is zinc di-2-ethylhexyldithiophosphate.

6. The lubricating oil composition of claim 1, wherein the zinc saturated aliphatic monocarboxylate is zinc 2-ethylhexanoate.

7. A lubricating oil composition comprising a zinc dithiophosphate and a zinc saturated aliphatic monocarboxylate dissolved in a base oil, wherein the zinc dithiophosphate and zinc saturated aliphatic monocarboxylate are in the ratio of 1:0.001 to 1:0.08 by weight, and the total amount of both zinc salts is in the range of 50 to 10,000 ppm expressed in terms of their zinc content.

8. The lubricating oil composition of claim 7, wherein the zinc dithiophosphate and zinc saturated aliphatic monocarboxylate are in the ratio of 1:0.008 to 1:0.07 by weight, and the total amount of both zinc salts is in the range of 100 to 1,000 ppm expressed in terms of their zinc content.