Viscosity Chart

Viscosity is one of the most important properties of hydraulic fluid. Viscosity is the measure of how a fluid resists flow. Viscosity is dependent on temperature. Viscosity increases (fluid is thicker) as temperature decreases. The viscosity of the fluid must be selected for the overall working temperature range of the application. Some trade-offs in selecting viscosity are as follows:

Can reduce internal leakage rates
Generally allows higher working pressures and loads
May cause pumps to cavitate during cold temperature start-up
May cause high pressure drops, resulting in additional power consumption
May cause sluggish or slow equipment operation, predominant in systems utilizing servo valves

Low Viscosity

May cause components to wear and fail sooner
Generally permits more efficient operating systems and wild generate less wasted heat in piping
May increase internal leakage and reduce volumetric efficiency of hydraulic components

Viscosity is expressed in centistokes (cSt), referred to as the fluid kinematic viscosity, or Saybolt Universal Seconds (abbreviated either SUS or SSU).

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Source: Lightening Reference Handbook, by Berendson Fluid Power, Inc, Tulsa, OK