Lubricating oil additive and lubricating oil composition containing same

A lubricating oil composition, comprising:

• a base oil;
• a molybdenum compound (A) represented by the following general formula (1);
• and
• a sulfur-based composition (B) containing a sulfur-based compound (b-1) represented by the following general formula (2) and at least one kind of sulfur-based compound (b-2) represented by the following general formula (3) or (4), wherein a mass ratio “b-1:b-2” between contents of the sulfur-based compound (b-1) and the sulfur-based compound (b-2) in the sulfur-based composition (B) is from 0.5:99.5 to 95:5:
• where R1 to R4 each represent linear or branched alkyl groups having 8 or 13 carbon atoms, the groups being identical to or different from each other, and X1 to X4 each independently represent an oxygen atom or a sulfur atom;
• where R5 and R6 each represent alkyl groups having 3 to 24 carbon atoms, the groups being identical to or different from each other, and “a” and “b” each represent a number from 1 to 5;
• where R7 to R10 each represent alkylene groups having 1 to 28 carbon atoms, the groups being identical to or different from each other, R11 and R12 each independently represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, “c” and “d” each independently represent a number from 1 to 5, and averages of “c” and “d” each are from 2.5 to 3.5, provided that at least one of R11 or R12 represents an alkyl group having 1 to 3 carbon atoms.

A method of sustaining a friction-reducing effect of a molybdenum compound (A) represented by the following general formula (1), comprising adding a sulfur-based composition (B) to a lubricating oil composition comprising a base oil and the molybdenum compound (A), wherein the sulfur-based composition (B) contains a sulfur-based compound (b-1) represented by the following general formula (2) and at least one kind of sulfur-based compound (b-2) represented by the following general formula (3) or (4), wherein a mass ratio “b-1:b-2” between contents of the sulfur-based compound (b-1) and the sulfur-based compound (b-2) in the sulfur-based composition (B) is from 0.5:99.5 to 95:5:

• where R1 to R4 each represent linear or branched alkyl groups having 8 or 13 carbon atoms, the groups being identical to or different from each other, and X1 to X4 each independently represent an oxygen atom or a sulfur atom;
• where R5 and R6 each represent alkyl groups having 3 to 24 carbon atoms, the groups being identical to or different from each other, and “a” and “b” each represent a number from 1 to 5;
• where R7 to R10 each represent alkylene groups having 1 to 28 carbon atoms, the groups being identical to or different from each other, R11 and R12 each independently represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, “c” and “d” each independently represent a number from 1 to 5, and averages of “c” and “d” each are from 2.5 to 3.5, provided that at least one of R11 or R12 represents an alkyl group having 1 to 3 carbon atoms.