Viscosity Index

Canonical Definition

A dimensionless number that expresses the rate of change of oil viscosity with temperature. Higher viscosity index (VI) means the oil maintains more consistent viscosity across a wider temperature range. Mineral oils typically have VI of 90–110; conventional hydraulic fluids 95–105; VI-improved engine oils 140–175; synthetic fluids 150–200+. Low viscosity index oils become very thin at high temperatures (risking inadequate film thickness) and very thick at low temperatures (risking cold-start pump cavitation and excessive restriction).

Engineering Context

Viscosity index is critical for mobile equipment operating across wide ambient temperature ranges. A hydraulic excavator operating from −20°C winter to +40°C summer ambient requires a hydraulic fluid with VI ≥ 160 to maintain acceptable pump inlet viscosity at cold start while avoiding excessive thinning at peak operating temperature. Synthetic base oils inherently deliver higher VI than mineral oils.

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