GB/T 2541-1981 "Calculation Table of Viscosity Index for Petroleum Products" The standard aims to provide a method for calculating the viscosity index of petroleum products. Viscosity index is a measure of the degree of change in viscosity of lubricating oils at different temperatures and is essential for evaluating the lubrication performance of oils over a wide Temperature range. The standard specifies the use of Optical inspection measurement and related formulas to calculate the viscosity index of petroleum products to ensure their Stability and Reliability under various operating conditions.