Viscosity Index (VI) Table of Contents Physical & Chemical Properties VI is a commonly used expression of an oil's change of viscosity with temperature. VI is based on two hypothetical oils with arbitrarily assigned VI's of 0 and 100. The higher the viscosity index the smaller the relative change in viscosity with temperature. Most industrial mineral lubricating oils have a VI between 55 and 100, but VI varies from 0 to "high VI" oils with VI up to 175. Viscosity-Temperature-VI relationship is shown in the following table: Industrial Oil Viscosity, cSt Viscosity, cSt Viscosity Visc-Temp ISOVG 32 40°C 100°C Index Coefficient Machine Oil 30.4 4.8 58 0.842 Turbine Oil 32 5.4 102 0.831 Hydraulic Oil 30.4 6.1 154 0.799 A less arbitrary indication of the change in viscosity with temperature is the viscosity temperature coefficient. For 40 to 100 degree C it is: Viscosity (cSt) at 40 degree C minus Viscosity (cSt) at 100 degrees C = C-1, divided by the Viscosity (cSt) at 40 degrees C. Calculated values of the viscosity-temperature coefficient are also shown in the table. The lower the value of the coefficient, the higher the VI. The coefficient for mineral oils can vary by a factor of 10 depending on the temperatures. For more information call 1-888-HERGUTH (437-4884)

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