For oils with a VI greater than 100, the calculation is somewhat more complex and can be found in ASTM 2270. Since the units of U, L, and H are the same, VI is a unitless number. For simplicity, ASTM D2270 provides a tabulation of the L and H values for a reference oil (U) with a known kinematic viscosity at 40 C and 100 C. VI is calculated by comparing the kinematic viscosity of the unknown oil at 40 C (U) (per ASTM D341), with the kinematic viscosity of two reference fluids with VIs of 0 (L) and 100 (H) where all three oils (U, L, and H) have the same kinematic viscosity at 100 C. For an oil with a VI < 100, the index can be calculated according to the formula shown in Fig.
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