Viscosity Index

The measure of the rate of change of viscosity with temperature. Heating tends to make lubricants thinner; cooling makes them thicker. The higher the viscosity index is for a particular fluid, the less of a change in viscosity there will be over a given temperature range. In determining the viscosity index, two temperatures of viscosity are taken, one at 104°F (40°C) and the other at 212°F (100°C).