Viscosity index reveals oil resistance to improving viscosity in cold and heat and decreasing it in heat. Oil is ideal to have a constant viscosity at different temperatures.
To achieve this ideal oil, they use improvers and viscosity boosters. These materials are polymers that disperse into dense molecules in the oil at low temperatures and compensate for the decrease in viscosity by dissolving in the oil at high temperatures. This additive is often used in multi-grade oils.