US20110077183A1

US20110077183A1 - Lubricant composition

Info

Publication number: US20110077183A1
Application number: US12/921,951
Authority: US
Inventors: Hideyuki Murata; Moritsugu Kasai
Original assignee: Idemitsu Kosan Co Ltd
Current assignee: Idemitsu Kosan Co Ltd
Priority date: 2008-03-17
Filing date: 2009-03-11
Publication date: 2011-03-31
Also published as: TWI467008B; JP5330716B2; BRPI0908558A2; WO2009116438A1; EP2256180A1; CN101970626A; TW201002814A; MY159387A; EP2256180A4; JP2009221382A

Abstract

A lubricating oil composition which comprises a base oil of lubricating oil having a kinematic viscosity of 3.5 to 10 mm²/s at 100° C. and a viscosity index of 100 or greater and (a) an ethylene-α-olefin copolymer having a number-average molecular weight of 2,500 to 25,000 and/or (b) a polymethacrylate having a number-average molecular weight of 10,000 to 30,000, wherein the kinematic viscosity of the composition is 5.6 to 15.0 mm²/s at 100° C. The lubricating oil exhibits excellent property for decreasing fuel consumption and can suppress generation of noise and vibration.

Description

TECHNICAL FIELD

The present invention relates to a lubricating oil composition and, more particularly, to a lubricating oil composition which suppresses generation of noise and vibration and, at the same time, can exhibit excellent property for decreasing fuel consumption when a mechanical apparatus filled with a lubricating oil such as an internal combustion engine is working.

BACKGROUND ART

Recently, production and use of automobiles and industrial machines are rapidly increasing worldwide, and the amount of consumed energy such as petroleum is increasing due to the increase. Therefore, the air pollution due to the use of the energy is increasing extremely, and this makes a serious problem.
To prevent the air pollution described above, it is necessary that exhaust gases discharged from automobiles and the like which are the main source of the air pollution be decreased. As the method for satisfying the above requirement from the standpoint of the lubricating oil, it is required that a decrease in the fuel consumption be achieved with the lubricating oil.
As for the method for decreasing fuel consumption with a lubricating oil such as a lubricating oil for internal combustion engines (an engine oil), it is known to be effective for decreasing friction loss (stirring resistance) with an engine oil that viscosity is decreased (a decrease in the viscosity) (for example, refer to Patent Reference 1).
However, when the viscosity of a lubricating oil is simply decreased, for example, by using a base oil having a small viscosity, a problem arises in that noise and vibration of the engine increase. Noise and vibration cause serious adverse effects on the living environment, i.e., the noise and vibration pollution.
Therefore, it is desired that a lubricating oil composition which has a decreased viscosity to exhibit the excellent property for decreasing fuel consumption and can suppress generation of noise and vibration is developed.

[Patent Reference 1] Japanese Patent Application Laid-Open No. 2004-137317

DISCLOSURE OF THE INVENTION

Problems to be Overcome by the Invention

Under the above circumstances, the present invention has an object of providing a lubricating oil composition which exhibits the excellent property for decreasing fuel consumption and can suppress generation of noise and vibration.

Means for Overcoming the Problems

As the result of intensive studies by the present inventors to develop the lubricating oil composition exhibiting the advantageous properties described above, it was found that the object could be achieved by using a composition which comprised a specific base oil and an ethylene-α-olefin copolymer and/or a polymethacrylate each having a specific molecular weight and had a kinematic viscosity adjusted at a specific value. The present invention has been completed based on the knowledge. The present invention provides:
[1] A lubricating oil composition which comprises a base oil of lubricating oil having a kinematic viscosity of 3.5 to 10 mm²/s at 100° C. and a viscosity index of 100 or greater and (a) an ethylene-α-olefin copolymer having a number-average molecular weight of 2,500 to 25,000 and/or (b) a polymethacrylate having a number-average molecular weight of 10,000 to 30,000, wherein a kinematic viscosity of the composition is 5.6 to 15.0 mm²/s at 100° C.;
[2] A lubricating oil composition described in [1], wherein the kinematic viscosity of the lubricating oil composition is 5.6 to 12.5 mm²/s at 100° C.;
[3] A lubricating oil composition described in any one of [1] and [2], wherein the kinematic viscosity of the base oil of lubricating oil is 3.5 to 6 mm²/s at 100° C.;
[4] A lubricating oil composition described in any one of [1] and [2], which further comprises at least one additive selected from antioxidants, extreme pressure agents, antiwear agents, oiliness improvers, detergent-dispersants, viscosity index improvers and pour point depressants; and
[5] A lubricating oil composition described in any one of [1] and [2], which is used for internal combustion engines.

THE EFFECT OF THE INVENTION

In accordance with the present invention, the lubricating oil composition which exhibits excellent property for decreasing fuel consumption and can suppress generation of noise and vibration can be provided.

THE MOST PREFERRED EMBODIMENT TO CARRY OUT THE INVENTION

For the base oil used for the lubricating oil composition of the present invention, it is necessary that a base oil having a kinematic viscosity of 3.5 to 10 mm²/s at 100° C. and a viscosity index of 100 or greater be used.
When the kinematic viscosity is smaller than 3.5 mm²/s at 100° C., generation of noise and vibration cannot be sufficiently prevented, occasionally. When the kinematic viscosity exceeds 10 mm²/s at 100° C., there is the possibility that the property for decreasing fuel consumption is adversely affected. From the above standpoints, it is preferable that the kinematic viscosity is 3.5 to 6 mm²/s at 100° C.
It is necessary that the base oil used in the present invention have a viscosity index of 100 greater. When the viscosity index is 100 or greater, the fuel consumption can be decreased by decreasing the viscosity of the composition at low temperatures, and the effect of suppressing noise and vibration can be enhanced since the decrease in the viscosity at higher temperatures can be suppressed.
From the above standpoints, it is preferable that the viscosity index of the base oil is 110 or greater and more preferably 120 or greater.
It is preferable that the base oil used in the present invention has a sulfur content of 1,000 ppm by mass or smaller and more preferably 500 ppm by mass or smaller. When the sulfur content is 1,000 ppm or smaller, stability of the composition can be enhanced.
The base oil used in the lubricating oil composition of the present invention is not particularly limited as long as the base oil satisfies the above requirements. Mineral oils and/or synthetic oils used for conventional lubricating oils can be used.
Examples of the mineral oil include oils obtained by treating a topping fraction of crude oil or a lubricating oil fraction, which is obtained by vacuum distillation of a topped crude obtained by the topping, with at least one of the treatments such as deasphalting with solvents, extraction with solvents, hydrocracking, hydrodewaxing, dewaxing with solvents and hydrorefining and base oils produced by isomerization of mineral-based wax or wax produced in accordance with the Fischer-Tropsch process (gas-to-liquid wax).
Examples of the synthetic oil include polybutene, hydrogenation products of polybutene, poly-α-olefins such as 1-decene oligomers, hydrogenation products of poly-α-olefins, diesters such as di-2-ethylhexyl adipate and di-2-ethylhexyl sebacate, polyol esters such as trimethylolpropane caprylate and pentaerythritol 2-ethylhexanoate, aromatic synthetic oils such as alkylbenzenes and alkylnaphthalenes, polyalkylene glycols and derivatives of polyalkylene glycols.
In the present invention, base oils of mineral oil, base oils of synthetic oil or mixtures prepared as desired from two or more types of oils selected from the above oils can be used as the base oil. Examples of the base oil include one or more types of base oils of mineral oil, one or more types of base oils of synthetic oil and mixed oils prepared from one or more types of base oils of mineral oil and one or more types of base oils of synthetic oil. It is preferable that a base oil of mineral oil obtained by a refining treatment including hydrocracking or a mixture of the base oil of mineral oil described above with a hydrogenation product of a poly-α-olefin such as a 1-decene oligomer is used.
In the present invention, base oils of mineral oil, base oils of synthetic oil or mixtures prepared from two or more types of oils selected from these base oils as desired can be used as the base oil.
In the present invention, as component (a), an ethylene-α-olefin copolymer having a number-average molecular weight of 2,500 to 25,000 is used.
When the number-average molecular weight is smaller than 2,500, the effect of suppressing noise and vibration is not sufficient. When the number-average molecular weight exceeds 25,000, stability under shearing is decreased, and it becomes difficult that the effect is maintained with stability. When the suppression of noise and vibration and the stability under shearing are considered, it is preferable that the number-average molecular weight of the ethylene-α-olefin copolymer is in the range of 2,500 to 5,000.
As the α-olefin used for the ethylene-α-olefin copolymer, α-olefins having 3 to 20 carbon atoms such as propylene, 1-butene and 1-decene are preferable. It is preferable that the ratio of the amounts of ethylene to α-olefin is in the range of 1:9 to 9:1 (the ratio of amounts by mass).
The ethylene-α-olefin copolymer of component (a) may be used singly or in combination of two or more.
In general, the amount of the ethylene-α-olefin copolymer is selected in the range of 0.1 to 20% by mass based on the amount of the entire composition. When the amount of the copolymer is 0.1% by mass or greater, the effect of suppressing noise and vibration can be obtained. When the amount of the copolymer is 20% by mass or smaller, the stability under shearing is excellent, and the effect of the copolymer can be maintained with stability without the problem of increase in the viscosity at low temperatures.
In the present invention, a polymethacrylate having a number-average molecular weight of 10,000 to 30,000 is used as component (b). The polymethacrylate may be any of the non-dispersion type or the dispersion type.
When the number-average molecular weight is smaller than 10,000, the effect of suppressing noise and vibration is not sufficient. When the number-average molecular weight exceeds 30,000, the stability under shearing decreases, and it is difficult that the effect is maintained with stability. When the suppression of noise and vibration and the stability under shearing are considered, it is preferable that the number-average molecular weight of the polymethacrylate is in the range of 15,000 to 25,000.
The polymethacrylate of component (b) may be used singly or in combination of two or more.
The amount of the methacrylate is selected in a manner such that the composition of the present invention satisfies the requirement for the kinematic viscosity at 100° C. In general, the amount is selected in the range of 0.1 to 10% by mass based on the amount of the entire composition. When the amount is 0.1% by mass or greater, the effect of decreasing the fuel consumption and the effect of suppressing noise and vibration can be obtained. When the amount is 10% by mass or smaller, the stability under shearing is excellent, and the effect can be maintained with stability without the problem of excessive increase in the viscosity at low temperatures.
The present invention provides the composition obtained by adding component (a) and/or component (b) to the base oil. Various additives conventionally added to lubricating oil compositions may be used in accordance with the object of the use.
For example, it is preferable that at least one additive selected from antioxidants, extreme pressure agents, antiwear agents, oiliness improvers, detergent-dispersants, viscosity index improvers other than components (a) and (b) and pour point depressants is added.
As the antioxidant, phenol-based antioxidants and/or amine-based antioxidants are used.
As the phenol-based antioxidant, suitable antioxidants can be selected as desired from phenol-based antioxidants conventionally used as the antioxidant for lubricating oils. Examples of the phenol-based antioxidant include 2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-hydroxymethyl-phenol, 2,6-di-tert-butylphenol, 2,4-dimethyl-6-tert-butylphenol, 2,6-di-tert-butyl-4-(N,N-dimethylaminomethyl)phenol, 2,6-di-tert-amyl-4-methylphenol, 4,4′-methylenebis(2,6-di-tert-butylphenol), 4,4′-bis(2,6-di-tert-butylphenol), 4,4′-bis(2-methyl-6-tert-butylphenol), 2,2′-methylene-bis(4-ethyl-6-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-tert-butyl-phenol), 4,4′-butylidenebis(3-methyl-6-tert-butylphenol), 4,4′-isopropyl-idenebis(2,6-di-tert-butylphenol), 2,2′-methylenebis(4-methyl-6-nonyl-phenol), 2,2′-isobutylidenebis(4,6-dimethylphenol), 2,2′-methylenebis(4-methyl-6-cyclohexylphenol), 2,4-dimethyl-6-tert-butylphenol, 4,4′-thio-bis(2-methyl-6-tert-butylphenol), 4,4′-thiobis(3-methyl-6-tert-butylphenol), 2,2′-thiobis(4-methyl-6-tert-butylphenol), bis(3-methyl-4-hydroxy-5-tert-butylbenzyl)sulfide, bis(3,5-di-tert-butyl-4-hydroxybenzyl)sulfide, 2,2′-thiodiethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], tridecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, pentaerythrityl tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], octyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, octadecyl-3-(3,5-di-tert-butyl-4-hydroxylphenyl)propionate and octyl-3-(3-methyl-5-tert-butyl-4-hydroxy-phenyl)propionate.
As the amine-based antioxidant, suitable antioxidants can be selected as desired from amine-based antioxidants conventionally used as the antioxidant for lubricating oils. Examples of the amine-based antioxidant include diphenylamine-based antioxidants such as diphenylamine; alkylated diphenylamines having alkyl groups having 3 to 20 carbon atoms such as monooctyldiphenylamine, monononyldiphenylamine, 4,4′-dibutyldiphenylamine, 4,4′-dihexyl-diphenylamine, 4,4′-dioctyldiphenylamine, 4,4′-dinonyldiphenylamine, tetrabutyldiphenylamine, tetrahexyldiphenylamine, tetraoctyldiphenyl-amine and tetranonyldiphenylamine; and naphthylamine-based antioxidants such as α-naphthylamine, phenyl-α-nathphthylamine and alkyl substituted phenyl-α-naphthylamines examples of which include butylphenyl-α-naphthylamine, hexylphenyl-α-naphthylamine, octyl-phenyl-α-naphthylamine and nonylphenyl-α-naphthylamine. Among these amine-based antioxidants, diphenylamine-based antioxidants are preferable to naphthylamine-based antioxidants from the standpoint of the effect. Alkylated diphenylamines having alkyl groups having 3 to 20 carbon atoms are preferable, and 4,4′-di(C₃˜C₂₀alkyl)diphenylamines are more preferable.
In the present invention, the phenol-based antioxidant may be used singly or in combination of two or more, and the amine-based antioxidant may be used singly or in combination of two or more. One or more types of the phenol-based antioxidants and one or more types of amine-based antioxidants may be used in combination.
It is preferable that the amount of the antioxidant is selected in the range of 0.05 to 3% by mass and more preferably in the range of 0.2 to 2% by mass based on the amount of the entire lubricating oil composition from the standpoint of the balance between the effect and the economy.
Examples of the extreme pressure agent and the antiwear agent include anti-wear agents containing sulfur such as zinc dithiophosphate, zinc dithiocarbamate, disulfides, sulfurized olefins, sulfurized oils and fats, sulfurized esters, thiocarbonates and thiocarbamates; antiwear agents containing phosphorus such as esters of phosphorous acid, esters of phosphoric acid, esters of phosphoric acid and amine salts and metal salts of these acids; and antiwear agents containing sulfur and phosphorus such as esters of thiophosphorous acid, esters of thiophosphoric acid, esters of thiophosphonic acid and amine salts and metal salts of these acids.
As the oiliness improver (the agent for adjusting friction), any desired compounds conventionally used as the agent for adjusting friction for lubricating oils can be used. Examples of the oiliness improver include aliphatic amines, esters of fatty acids, amides of fatty acids, fatty acids, aliphatic alcohols and aliphatic ethers, which have at least one alkyl group or alkenyl group having 6 to 30 carbon atoms and, preferably, at least one linear alkyl group or linear alkenyl group having 6 to 30 carbon atoms in the molecule. The amounts of the extreme pressure agent, the antiwear agent and the oiliness improver are each, in general, in the range of 0.01 to 3% by mass and preferably in the range of 0.1 to 1.5% by mass.
Examples of the detergent-dispersant include metal-based detergents such as neutral or perbasic sulfonates, phenates, salicylates, carboxylates and phosphonates having alkaline earth metals such as Ca, Mg and Ba. Among these detergents, metal-based detergents such as Ca sulfonate, Ca salicylate and Ca phenate are preferable, and perbasic (having a base value of 150 to 500 mg KOH/g in accordance with the perchloric acid method) Ca sulfonate, Ca salicylate and Ca phenate are more preferable.
Examples of the ashless dispersant include imides of succinic acid (including products of boration) and esters of succinic acid.
The detergent-dispersant is used, in general, in an amount in the range of 0.01 to 10% by mass and preferably in the range of 0.1 to 5% by mass.
Examples of the viscosity index improver other than components (a) and (b) include polymethacrylates other than the polymethacrylates described above, olefin-based copolymers other than the olefin-based copolymers described above, olefin-based copolymers of the dispersion type and styrene-based copolymers (such as styrene-diene copolymers and styrene-isoprene copolymers). The amount of the viscosity index improver is, in general, about 0.5 to 15% by mass and preferably 1 to 10% by mass.
In the present invention, pour point depressants, defoaming agents and surfactants can also be used.
It is necessary that the lubricating oil composition of the present invention has a kinematic viscosity of 5.6 to 15.0 mm²/s at 100° C. When the kinematic viscosity is smaller than 5.6 mm²/s at 100° C., generation of noise and vibration cannot be sufficiently prevented, occasionally. When the kinematic viscosity exceeds 15.0 mm²/s at 100° C., there is the possibility that the property for decreasing fuel consumption is adversely affected. From the above standpoints, it is preferable that the kinematic viscosity is 5.6 to 12.5 mm²/s at 100° C.
The lubricating oil composition of the present invention exhibits the excellent property for decreasing fuel consumption and the effect of suppressing generation of noise and vibration.
For example, when this lubricating oil composition is used for an internal combustion engine, the composition is useful as the oil for internal engines (the engine oil) exhibiting the property for decreasing fuel consumption, and generation of noise and vibration during working of the engine, in particular, during working of the engine for acceleration can be suppressed. Therefore, the lubrication oil composition is very advantageously used as the lubricating oil for internal engines which are used for four-wheeled vehicles and two-wheeled vehicles having a 4-cycle engine.

EXAMPLES

The present invention will be described more specifically with reference to examples in the following. However, the present invention is not limited to the examples.
Noise and the property for decreasing fuel consumption were evaluated in accordance with the following methods.

(1) Evaluation of Noise

Noise was measured using an engine motoring apparatus in accordance with the methods for measuring noise which are described in the following.

[Engine Motoring Apparatus and Condition of Operation]

Used engine: a water-cooled 600 cc 4-cylinder engine
Valve style: DOHC (straight)
Number of rotation of the engine: 3,000 rpm
Oil temperature at the oil pan: 100° C.
Motor for driving: 7.5 kW

[Method for Measuring Noise]

Using a noise meter (manufactured by ONO SOKKI Co., Ltd.; “LA5560”), the power spectrum at 6,300 Hz was measured by a frequency analyzer (manufactured by ONO SOKKI Co., Ltd.; REPOLYZER XN-8100).

(2) Evaluation of Property for Decreasing Fuel Consumption

The property for decreasing fuel consumption was evaluated by measuring the torque (Nm) on the motor for driving the engine using the engine motoring apparatus used for measuring noise described above in (1).

[Engine Motoring Apparatus and Condition of Operation]

Used engine: a water-cooled 600 cc 4-cylinder engine
Valve style: DOHC (straight)
Number of rotation of the engine: 5,000 rpm
Oil temperature at the oil pan: 100° C.
Motor for driving: 7.5 kW

Examples 1 to 5 and Comparative Examples 1 to 3

Lubricating oils were prepared using base oils and additives shown in Table 1, and noise and the property for decreasing fuel consumption were evaluated in accordance with the methods described above. The results are shown in Table 1.

TABLE 1

	Example	Comparative Example

	1	2	3	4	5	1	2	3

Formulation (% by mass)

Base oil A of lubricating oil ¹⁾	65.8	85.8	67.3	—	67.3	66.8	54.8	—
Base oil B of lubricating oil ²⁾	20.0	—	20.0	86.8	20.0	20.0	20.0	—
Base oil C of lubricating oil ³⁾	—	—	—	—	—	—	—	84.8
Polymethacrylate A ⁴⁾	—	2.0	1.5	1.0	1.5	3.0	15.0	4.0
Polymethacrylate B ⁵⁾	—	—	—	—	1.0	—	—	—
zinc dialkyldithiophosphate ⁶⁾	1.1	1.1	1.1	1.1	1.1	1.1	1.1	1.1
metal-based detergent ⁷⁾	4.0	4.0	4.0	4.0	4.0	4.0	4.0	4.0
Succinimide modified with	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0
boron A ⁸⁾
Succinimide modified with	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0
boron B ⁹⁾
polybutenylsuccinimide ¹⁰⁾	2.1	2.1	2.1	2.1	2.1	2.1	2.1	2.1
antioxidant ¹¹⁾	0.8	0.8	0.8	0.8	0.8	0.8	0.8	0.8
Ethylene-α-olefin copolymer	4.0	2.0	—	2.0	—	—	—	—
A ¹²⁾
Ethylene-α-olefin copolymer	—	—	1.0	—	—	—	—	1.0
B ¹³⁾
other additive ¹⁴⁾	0.2	0.2	0.2	0.2	0.2	0.2	0.2	0.2
Kinematic viscosity of base oil	4.64	4.47	4.64	5.28	4.64	4.64	4.64	10.90
at 100° C., mm²/s
Kinematic viscosity of composition	7.80	7.30	7.05	8.23	7.10	7.22	15.50	15.40
at 100° C., mm²/s

Evaluation

noise, dB	75.8	76.2	76.3	74.8	76.3	78.3	75.2	76.0
property for decreasing fuel	19.8	19.6	19.6	19.8	19.6	19.7	20.6	20.5
consumption, Nm

[Notes]
¹⁾Paraffinic mineral oil; the kinematic viscosity at 100° C.: 4.469 mm²/s; the viscosity index: 127
²⁾Paraffinic mineral oil; the kinematic viscosity at 100° C.: 5.285 mm²/s; the viscosity index: 104
³⁾Paraffinic mineral oil; the kinematic viscosity at 100° C.: 10.89 mm²/s; the viscosity index: 107
⁴⁾The number-average molecular weight: 300,000
⁵⁾The number-average molecular weight: 21,000
⁶⁾A zinc dialkyldithiophosphate of the secondary alkyl type; the content of P: 8.6% by mass
⁷⁾Ca sulfonate; the base value (the perchioric acid method): 300 mg KOH/g; the content of Ca: 12.5% by mass
⁸⁾The content of nitrogen: 1.8% by mass; the content of boron: 2.0% by mass
⁹⁾The content of nitrogen: 2.3% by mass; the content of boron: 1.9% by mass
¹⁰⁾The content of nitrogen: 1.0% by mass; the content of boron: 0% by mass
¹¹⁾A dialkyldiphenylamine
¹²⁾The number-average molecular weight: 2,600; the kinematic viscosity at 100° C.: 600 mm²/s; the viscosity index: 240
¹³⁾The number-average molecular weight: 3,700; the kinematic viscosity at 100° C.: 2,000 mm²/s; the viscosity index: 300
¹⁴⁾A defoaming agent

It is shown by the results in Table 1 that Ethylene-α-olefin copolymer A or B having a number-average molecular weight of 2,600 or 3,700, respectively, was used in Examples 1 to 4, and a polymethacrylate having a number-average molecular weight of 21,000 was used in Example 5. In all Examples, the properties of the base oil and the kinematic viscosity of the composition at 100° C. satisfied the requirements. Therefore, the lubricating oil compositions of the present invention exhibited the excellent property for decreasing noise and the excellent property for decreasing fuel consumption.
In contrast, in Comparative Examples 1 and 2 in which none of the ethylene-α-olefin copolymer and the polymethacrylate having the number-average molecular weight required in the present invention was used, either one of the property for decreasing noise and the property for decreasing fuel consumption was insufficient. In Comparative Example 3 in which the ethylene-α-olefin copolymer having the number-average molecular weight of 3,700 was used but the kinematic viscosity of the composition was 15.4 mm²/s at 100° C., the property for decreasing fuel consumption was poor.

INDUSTRIAL APPLICABILITY

The lubricating oil composition of the present invention exhibits excellent property for decreasing fuel consumption and can suppress generation of noise and vibration when the lubricating oil composition is used for various mechanical apparatuses such as internal combustion engines. Therefore, the lubricating oil composition is useful as the lubricating oil which can prevent the air pollution and the noise and vibration pollution.

Claims

1. A lubricating oil composition which comprises a base oil of lubricating oil having a kinematic viscosity of 3.5 to 10 mm²/s at 100° C. and a viscosity index of 100 or greater and (a) an ethylene-α-olefin copolymer having a number-average molecular weight of 2,500 to 25,000 and/or (b) a polymethacrylate having a number-average molecular weight of 10,000 to 30,000, wherein a kinematic viscosity of the composition is 5.6 to 15.0 mm²/s at 100° C.

2. The lubricating oil composition according to claim 1, wherein the kinematic viscosity of the lubricating oil composition is 5.6 to 12.5 mm²/s at 100° C.

3. The lubricating oil composition according to claim 1, wherein the kinematic viscosity of the base oil of lubricating oil is 3.5 to 6 mm²/s at 100° C.

4. The lubricating oil composition according to claim 1, which further comprises at least one additive selected from the group consisting of an antioxidant, an extreme pressure agent, an antiwear agent, an oiliness improver, a detergent-dispersant, a viscosity index improver and a pour point depressant.

5. (canceled)

6. An internal combustion engine comprising the lubricating oil composition according to claim 1.