Selection of viscosity must be made with the normal operating temperature of the machine in mind. The temperature is usually measured at the pump. The correct viscosity at the expected operating temperature will allow the fluid to transmit the power most efficiently while still lubricating and cooling the machine without exhibiting excessive leakage. If the viscosity is too high (the fluid is too thick), the operation of the machine will be sluggish and excessive heat will be generated as the fluid flows through the system. If the fluid is not viscous enough, the fluid will bypass the closefitting parts of components. This leakage is called slippage. As the fluid leaks internally from areas of high pressure to areas of lower pressure, heat will be generated. External leakage may occur as well, as the fluid may be thin enough to bypass seals.
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