| abstract |
A hydraulic oil composition obtained by blending a base oil having a sulfur content of less than 0.03% by mass, a content of saturates of 90% by mass or above, a viscosity index of 80 or above and an absolute viscosity at 40°C of 1 to 1000mPa s with (A) 1 to 20% by mass of a polymethacrylate-type viscosity index improver having a weight-average molecular weight of 10,000 to 50,000, (B) 0.005 to 5% by mass of an acid amide, (C) 0.005 to 5% by mass of (c-1) a specific hydrogenphosphate ester and/or (c-2) a specific hydrogenphosphite ester and (D) 0.005 to 5% by mass of at least one member selected from among polyhydric alcohol esters, alkanolamines, olefin sulfides, and thiocarbamates. The hydraulic oil composition exerts satisfactory and well-balanced properties essential to hydraulic oil which include energy saving, heat stability (sludge resistance), prevention of wear and scuffing, water shedding, and friction reduction in sliding parts. |