What does a viscosity index number mean?
The viscosity index (VI) number is a measure of the relative change in viscosity of oil over a temperature range. The HIGHER the viscosity index, the SMALLER the viscosity change over temperature. The VI is not related to the actual viscosity or SAE viscosity, but is a measure of the rate of viscosity change. The VI number is typically used only as an indicator. The actual performance results of low-temperature pumpability tests and high-temperature wear tests of a motor oil are better predictors of good performance in an engine. Generally, multigrade oils (0W-40, 10W-30, etc.) will have high viscosity indexes. Monograde oils (SAE 30, 40, etc.) will have lower viscosity indexes.
