Definition
A viscosity index (VI) improver — also called a viscosity modifier — is a polymer additive that reduces the rate at which an oil’s viscosity changes with temperature. Without VI improvers, mineral base oils thin dramatically at high temperatures and thicken excessively at low temperatures.
VI improvers work by a coil-stretch mechanism: at low temperatures, the polymer chains remain coiled and contribute little to viscosity; at high temperatures, they uncoil and occupy more volume, thickening the oil and partially compensating for the natural viscosity decrease.
Common polymer types:
- Olefin copolymers (OCP) — ethylene-propylene copolymers; most widely used, cost-effective
- Polymethacrylates (PMA) — excellent low-temperature performance; used in ATF, hydraulic fluids, and PCMO
- Styrene-maleic esters — used in some ATF applications
- Hydrogenated styrene-diene (HSD) — star polymers with high shear stability