Viscosity is a rating of a liquid’s resistance to fluid shear but is also a measure of its resistance to flow. Petroleum oils tend to thin as temperature increases and begin to gel at lower temperatures. In a hydraulic circuit, if the viscosity of hydraulic fluid is too low (fluid is thin), the chances of leakage past seals and poor surface lubrication increase. If viscosity is too high (fluid is thick), more pump force is required to push fluid through the circuit and the operation of actuators may be sluggish.

There are two measures of viscosity: absolute (also known as dynamic) and kinematic, which factors in the density of the oil. In hydraulics, kinematic viscosity tends to be used to rate oils.

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