Patent ID: 8735335

Claim:
A method for lubricating a hydraulic system operating at low temperature having an oil filter comprising the step of: supplying a hydraulic oil composition to the hydraulic system operating at low temperature; wherein said hydraulic oil composition is filtered with the oil filter whose pore diameter is 50 μm or less, and the hydraulic oil composition comprises: a mineral base oil (A) having kinematic viscosity at 100° C. of 1.5 to 5 mm 2 /s, pour point of −50° C. to −10° C., viscosity index of 100 or more, % C P of 70 or more, % C A of 2 or less, and aniline point of 106° C. or higher, containing tertiary carbon atoms at a ratio of 7.4 to 10% to the total carbon atoms of the mineral base oil (A), and being treated by catalytic dewaxing process; a mineral base oil (B) having kinematic viscosity at 100° C. of 1.5 to 6 mm 2 /s, and aniline point of lower than 106° C.; and a poly (meth)acrylate additive, and the content of the base oil (B) to the total amount of the base oil (A) and the base oil (B) is 5 to 40 mass %.