Viscosity index explained: how oils hold viscosity across temperature

The viscosity index (VI) is a single number describing how much a fluid’s viscosity changes with temperature — a high VI thins less as it warms. ASTM D2270 derives VI from kinematic viscosities at 40 °C and 100 °C.

Why the whole curve helps

VI compresses behavior to two points. Measuring the full viscosity–temperature curve at 0.1 °C resolution shows where a fluid actually deviates — valuable for multigrade oils and additive screening.

From curve to index

With a dense dataset in hand, computing VI and related metrics is straightforward, and you keep the underlying curve for everything else.

Map your oil across temperature →

See also: oil viscosity

See also: Oil and lubricant viscosity testing from −30 °C to +40 °C

What does a viscosity index of 100 mean?

A viscosity index of 100 is the reference point on the ASTM D2270 scale, not a viscosity measurement or a universal quality threshold. VI is dimensionless; the underlying measurements remain kinematic viscosities reported in mm²/s at 40 °C and 100 °C.

For QC, compare the calculated VI with the product specification and retain the paired viscosity results, since two oils can have the same VI while differing substantially in viscosity at both test temperatures.

What is a good viscosity index for an oil?

A good viscosity index is the value required for the oil’s formulation and operating-temperature range, so there is no single pass value for every lubricant. Engine oils, hydraulic fluids, gear oils and base stocks may have different ASTM D2270 requirements.

Set the acceptance limit against a defined product specification and verify it from the measured 40 °C and 100 °C viscosities in mm²/s; a VI result without those measurements is not sufficient for traceable QC.