Short answer
Viscosity index (VI) is a unitless number calculated from an oil’s kinematic viscosity at 40 °C and 100 °C, using ASTM D2270. A higher VI means the oil’s viscosity changes less as temperature changes. Base oil type sets most of it, and polymer viscosity index improvers can raise it further.
How VI is calculated
The lab measures kinematic viscosity at 40 °C and 100 °C (ASTM D445) and plugs both into the ASTM D2270 formula, which compares the oil against reference oils on a fixed scale. The scale was originally anchored to two reference oil series assigned 0 and 100, and modern oils commonly exceed 100.
Because VI comes from two viscosity measurements, it summarizes the slope of the viscosity-temperature curve rather than any single temperature.
What sets an oil’s VI
Base oil matters most. In API’s base oil groups, Group II stocks fall between 80 and 120 VI, Group III stocks are 120 or higher, and synthetic polyalphaolefins (Group IV) are typically high as well.
Formulators add polymer viscosity index improvers to build multigrade engine oils and multi-viscosity hydraulic fluids. Those polymers can be broken down by shear over time, which is why shear stability is tested alongside VI.
Where VI makes a practical difference
Equipment that starts cold and runs hot, such as outdoor hydraulic systems, mobile equipment and multigrade engine oils, benefits from oil that thins less when hot and thickens less when cold. ISO 6743-4 even uses VI to separate HM hydraulic oils from higher-VI HV oils.
Indoor equipment that runs at a steady temperature, like many plant gearboxes and compressors, is less sensitive, so a very high VI matters less there.
Frequently asked questions
Is VI the same as viscosity grade?
No. Viscosity grade tells you how thick an oil is at a reference temperature; VI tells you how much that thickness changes with temperature.
Can VI drop in service?
It can, if the polymer improvers in the oil shear down. Used-oil analysis that measures viscosity at both temperatures can show it.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].