VISCOSITY INDEX IMPROVING AGENT, AND LUBRICATING OIL COMPOSITION

摘要

PROBLEM TO BE SOLVED: To provide a viscosity index improving agent high in the viscosity index improving effect and making the low-temperature viscosity of a lubricating oil composition to hardly rise.SOLUTION: A viscosity index improving agent contains a (co)polymer (A) comprising as essential constituting monomers methyl (meth)acrylate and a polymerizable unsaturated group-containing polymer (Y), wherein (Y) is a polymer comprising as a constituting monomer at least one polymerizable unsaturated group-containing hydrocarbon compound (x); the average value of the numbers of polymerizable unsaturated groups of (Y) is 0.8 to 1.5; and the solubility parameter of (A) is 7.3 to 9.5 (cal/cm). There is also provided a lubricating oil composition containing the viscosity index improving agent and a base oil.