Viscosity index: It is defined as rate of change of viscosity with respect to temperature.

Significance:

• It is the most important property which determines the performance of lubricating oils under operating conditions.
• Viscosity index used to characterize the viscosity-temperature behaviour of lubricating oils.
• The lower the Viscosity index, the more the viscosity is affected by changes in temperature.
• On machine part moving at slow speeds under high pressures, a heavy oil should be used as it better resists being squeezed out from between the rubbing parts light oils can be used for lower pressures and high speeds.
• It is not possible to maintain a liquid oil film between two moving or sliding surfaces if the viscosity is too low.