There is disclosed a hydraulic working oil composition which comprises in the form of a blend, a base oil having a CA content of at most 5%; 0.01 to 5% by weight of an (A) aminic antioxidant such as 4,4'-dioctyldiphenylamine; 0.01 to 5% by weight of a (B) phenolic antioxidant such as 2,6-di-tert-butyl-4-methylphenol; 0.01 to 5% by weight of a (C) phosphoric ester such as tricresyl phosphate; and 0.001 to 5% by weight of a (D) fatty acid amide such as isostearic acid triethylenetetramide and/or polyhydric alcohol ester such as sorbitan monooleate each based on the whole amount of the composition. According to the present invention, there is provided the hydraulic working oil composition capable of effectively preventing premature deterioration of the working oil accompanying the trend towards high pressure and the sludge formation therein, being serviceable for a long period of time and at the same time, exhibiting stable working characteristics by eliminating the phenomenon of cylinder waviness, that is, a phenomenon of causing vibration and/or abnormal noise between a cylinder and a piston, between a rod and a (sealing) guide, or the like in a hydraulic working portion of hydraulic machinery.
展开▼
