The viscosity index of an oil or of the base oil of a grease is calculated from its kinematic viscosity at 40 °C and 100 °C. The higher the viscosity index, the less the viscosity changes with temperature — a high viscosity index means consistent lubrication across a wide operating range.
Calculated to ASTM D2270 / ISO 2909. Valid for a kinematic viscosity at 100 °C of 2 mm²/s and above, with the viscosity at 40 °C higher than the viscosity at 100 °C. Results are for information; they do not replace a test report.
| Below 35 | Low — naphthenic and untreated stocks |
| 35 – 80 | Medium |
| 80 – 110 | High — solvent-refined mineral oils |
| 110 – 150 | Very high — hydrocracked, PAO, ester |
| Above 150 | Exceptional — synthetics, PAG, VI-improved |
Two reference oils are defined for the same viscosity at 100 °C: one of viscosity index 0 (its viscosity at 40 °C is L) and one of viscosity index 100 (H). The oil under test, with viscosity U at 40 °C, is placed on that scale.
When U is at or above H the index is (L − U) ÷ (L − H) × 100. When U is below H the index is above 100 and the standard's extended calculation is used. L and H come from the standard's reference values; above 70 mm²/s at 100 °C they are calculated from the standard's equations.
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