Viscosity is a measure of an oil's internal resistance to flow and viscosity index is a measure of an oil's ability to resist change in viscosity as the temperature changes.

Viscosity is a measure of an oil's resistance to emulsification and viscosity index is a measure of an oil's ability to resist change in viscosity as the temperature changes.

Viscosity is a measure of an oil's ability to resist oxidation and viscosity index is a measure of an oil's ability to resist change in viscosity as the temperature changes.

Viscosity is a measure of an oil's lubricity and viscosity index is a measure of an oil's ability to resist change in lubricity as the pressure changes.