Lubricating oil additive and lubricating oil composition containing same

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 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 5:95 to 95:5, and
• wherein a content of a molybdenum element in the lubricating oil composition is from 200 ppm by mass to 2,000 ppm by mass, and a content of a sulfur element therein is from 500 ppm by mass to 8,000 ppm by mass:
• where R1 to R4 each represent alkyl groups having 6 to 18 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 represents an alkyl group having 1 to 22 carbon atoms, R6 represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, R7 represents a hydrogen atom or an alkyl group having 1 to 20 carbon atoms, and “a” represents a number from 1 to 10;
• where R8 and R9 each represent alkylene groups having 6 to 24 carbon atoms, the groups being identical to or different from each other, R10 and R11 each represent alkylene groups having 6 to 24 carbon atoms, the groups being identical to or different from each other, R12 and R13 each independently represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms, and “b” and “c” each independently represent a number from 1 to 8, provided that at least one of R12 or R13 represents an alkyl group having 1 to 3 carbon atoms.