When system working environment of high temperature, the higher viscosity of the oil, on the contrary, the low viscosity oil, higher working pressure, with higher viscosity, because at high pressure, the sealing is difficult, leakage is main problem, otherwise, the system working pressure is low, suitable for low viscosity oil, when the working parts of the hydraulic system is operating at a high speed, oil and liquid flow rate is high, then the pressure loss will also increase, and leakage decreases relatively, low viscosity oil should be used. On the contrary, the movement of working parts, the speed is low, should the oil viscosity is higher.