Viscosity index improvers, or viscosity modifiers, are polymeric molecules such as polyalkyl methacrylates (PAMA) and olefin copolymers (OCP), that are added to oil formulations to thicken, as well as adjust the viscosity-temperature relationship of the fluid. These polymers are added to resist change in engine oil viscosity with temperature, as oils have a tendency to thin at high temperature and thicken at lower temperatures.

By minimising this change in oil viscosity, viscosity modifiers allow lubricant manufacturers to expand and optimise the operating temperature range and conditions that the oil is likely to face. Viscosity index improvers are used to formulate multigrade oils such as SAE 15W-40 and 5W-30.

Fun With Syrup - Studies In Viscosity