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Calculate VI from kinematic viscosity at 40 °C and 100 °C, with a temperature–viscosity curve.
Inputs: U = KV@40°C, Y = KV@100°C (both in cSt) L and H are reference viscosities from ASTM D2270 Table 1
(piecewise quadratics in Y):
L = A·Y² + B·Y + C
H = D·Y² + E·Y + F If U > H (VI < 100): VI = 100 × (L − U) ÷ (L − H)
If U ≤ H (VI ≥ 100): N = (log H − log U) ÷ log Y
VI = (10^N − 1) ÷ 0.00715 + 100 Viscosity–temperature curve per ASTM D341 (Walther):
W(v) = log(log(v + 0.7)) = A − B·log(T/K)Valid for KV100 ≥ 2 cSt. Base-oil group labels are indicative, not a substitute for API base-stock classification.
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