Lubricating oil composition

1. A lubricating oil composition, comprising:

a base oil (A);

at least one ash-free additive (B) selected from the group consisting of an ash-free sulfur-based antioxidant (B1) and a hindered amine compound (B2), wherein the ash-free sulfur-based antioxidant (B1) is at least one selected from the group consisting of a thiocarbamate compound, a sulfur-containing triazine compound and sulfurized oils and fats, and the hindered amine compound (B2) has one or two piperidine-derived skeletons;

a boronated imide-type dispersant (C), wherein a sulfated ash content is 0.2% by mass or less; and

a non-boronated imide-type dispersant (D), wherein

the content of the ash-free additive (B) is 1.2% by mass or less based on the total amount of the lubricating oil composition, provided that in the case where the ash-free additive (B) contains the hindered amine compound (B2), the content of the hindered amine compound (B2) is less than 1.0% by mass based on the total amount of the lubricating oil composition, and

the content of the boron atom derived from the boronated imide-type dispersant (C) is 200 ppm by mass or more based on the total amount of the lubricating oil composition.

2. The lubricating oil composition according to claim 1 , wherein the sulfated ash content is 0.01% by mass or more and 0.2% by mass or less.

3. The lubricating oil composition according to claim 1 , wherein:

the content of the boron atom derived from the boronated imide-type dispersant (C) relative to the total content of the nitrogen atom derived from the boronated imide-type dispersant (C) and the nitrogen atom derived from the non-boronated imide-type dispersant (D) is 0.10 to 1.0 as a ratio by mass.

4. The lubricating oil composition according to claim 1 , further comprising at least one metal-based detergent (E) selected from the group consisting of a calcium-based detergent (E1) and a magnesium-based detergent (E2), wherein:

the content of the metal atom derived from the metal-based detergent (E) is 50 to 200 ppm by mass based on the total amount of the lubricating oil composition.

5. The lubricating oil composition according to claim 1 , further comprising a zinc dithiophosphate (F), wherein:

the content of the phosphorus atom derived from the zinc dithiophosphate (F) is 50 to 300 ppm by mass based on the total amount of the lubricating oil composition.

6. A method of lubrication, the method comprising:

applying the lubricating oil composition according to claim 1 to a gas engine equipped in a gas cogeneration system or equipped in a gas heat pump.

7. The lubricating oil composition according to claim 1 , wherein the thiocarbamate compound is represented by at least one formula selected from the group consisting of (b1-1a) and (b1-1b),

where R 11B , R 12B , R 13B and R 14B each represent an alkyl group having 1 to 30 carbon atoms, or a phenyl group,

R 11B , R 12B , R 13B and R 14B may be the same or different,

R 15B represents an alkylene group having 1 to 10 carbon atoms,

R 16B and R 17B each represent an alkyl group having 1 to 30 carbon atoms, or a phenyl group,

R 16B and R 17B may be the same or different, and

R 18B represents a hydrogen atom or an alkylene group having 1 to 10 carbon atoms.

8. The lubricating oil composition according to claim 7 , wherein the polysulfide compound includes at least one dihydrocarbyl polysulfide compound represented by a formula (b1-3),

R 21 —S x —R 22   (b1-3),

where R 21 and R 22 each independently represent a hydrocarbon group selected from an alkyl group, linear or branched, having 3 to 20 carbon atoms, an aryl group having 6 to 20 carbon atoms, an alkylaryl group having 7 to 20 carbon atoms, an arylalkyl group having 7 to 20 carbon atoms, and an alkenyl group, linear or branched, having 3 to 20 carbon atoms,

R 21 and R 22 may be the same or different, and

x represents an integer of 2 to 10.