A high-molecular-weight polymer, such as an olefin copolymer or a polymethacrylate, whose coils swell in hot oil and tighten in cold oil, so it adds more viscosity at high temperature. VI improvers make multigrade engine oils and high-VI hydraulic fluids possible. Their weakness is shear: gears, pumps and tight clearances cut the molecules, and in-service viscosity drops permanently, which oil analysis sees as a steady slide in KV100.
Viscosity's down 15 percent with no fuel in it, the viscosity index improver has sheared out.
Also heard as
- VII
- VI improver
- viscosity modifier
Related terms
Viscosity index
Number showing how little an oil's viscosity changes with temperature; higher means flatter.
Viscosity
A fluid's resistance to flow, the lubricant property that sets how thick an oil film can build.
Tackifier
Polymer additive that makes a lubricant sticky and stringy so it clings to chains, open gears and slides.
Additive package
Pre-blended mix of performance chemicals that a blender adds to base oil to make a finished lubricant.