What does a high viscosity index tell about a lubricating fluid?

A high viscosity index indicates that a lubricating fluid's viscosity remains relatively stable despite fluctuations in temperature.12

In the field of lubrication, viscosity is the most critical property because it determines the amount of friction between sliding surfaces and ensures a film is thick enough to prevent wear from solid-to-solid contact. Because the viscosity of most liquids drops significantly as temperatures rise, a high viscosity index is desirable; it signifies that the fluid experiences minimal changes in viscosity when temperatures shift, which helps keep variations in friction at a minimum.12