Internal resistance of moving is the viscosity of the lubricant or lube oil. Generally in case of lubricant viscosity is measured by a unit less parameter named viscosity index. Actually it is the major oil life indicator of lubricants.
Viscosity Index: Viscosity index is nothing but a unit less measuring system of change of viscosity with temperature change. To get clear vision about it have to know first, what is viscosity? Actually it is internal resistance of fluid by which we can understand either the fluid move towards fast or slow, or how much fast or how much slow. On the other hand it is force of fluid with works against the movement of the substance. For easy understanding we can say that, when we pouring two or more fluid from a bottle towards down, then which fluid down faster that is low viscous and slow mover fluid is high viscous.In some cases viscosity index of the lube oil can be improved by adding different types of additives and these additives called VI improver.
Viscosity Index can improve the productivity of lube oil in different ways.Because viscosity index is the key factor of lubricants stability in engine.In time of internal combustion of engine when lubricant works for the heat & friction reduce it losses viscosity but in case of high viscosity oil this losing rate is low.