A dimensionless score, calculated under ASTM D2270 from the kinematic viscosity at 40 C and 100 C, that rates how much an oil thins as it heats. Group I mineral oils sit near 95 to 105, Group III and PAO base oils run 120 to 140 or more, and polymer-boosted multigrade and high-VI hydraulic oils can pass 150. High VI pays off in equipment that sees wide temperature swings, such as mobile hydraulics and outdoor gearboxes.
Switch the loader fleet to a high viscosity index hydraulic and the cold-morning pump cavitation goes away.
Also heard as
- VI
Related terms
Viscosity
A fluid's resistance to flow, the lubricant property that sets how thick an oil film can build.
Base oil group
Five-group system that sorts base stocks by saturates, sulfur, viscosity index and chemical type.
Viscosity index improver
Polymer additive that thickens oil more when hot than when cold, raising its viscosity index.
PAO
Polyalphaolefin, the synthetic hydrocarbon base oil built from alpha-olefins and classed as Group IV.