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 →