Chemical additive that is added to finished lubricants to improve the viscosity index.
Lubricant additive, usually a high-molecular-weight polymer, that reduces the tendency of an oil to change viscosity with temperature. Multi-grade oils, which provide effective lubrication over a broad temperature range, usually contain viscosity index improvers.
While viscosity index improvers can enhance viscosity index, they can break down under shear or over time, resulting in diminished performance.
