The use of a copolymer of monomers.

It PRESENTS A lubricating oil comprising a copolymer of monomers as essential ingredients HAS A compound represented by formula (I) and a maleic acid or maleic anhydride, maleic acid salt, and an ester of maleic acid. In formula (I) Z represents a residue of a compound having 2 to 8 hydroxyl groups, AO represents one or more kinds of oxyalkylene groups TAKING EACH between 2 and 18 carbon atoms, with the proviso that IF there are two or more kinds of oxyalkylene groups, oxyalkylene GROUPS OF EACH OTHER STATES ARE BOTH IN A BLOCK AS A provision SHUFFLE. R represents an alkenyl group having 2 to 5 carbon atoms; R1 represents a hydrocarbon or a group AZIL having 1 and 24 carbon atoms; YA (or equal) 0, B (≥) 0, C (≥) 0, GAMMA is a positive integer, m and n independently are 0 or a positive integer, with the proviso that GAMMA + M + N = 2 A 8 N / (M + GAMMA) (smaller or equal) 1/2, AND GAMMA + BM + CN = 1 to 1,000.