The ASTM viscosity index and other systems for classifying lubricating oils

Even though the ASTM V.I. is based on an arbitrary and inconsistent series of reference oils, the system still persists as an industry standard to rate the viscosity-temperature characteristics of lubricating oils. There is need for an industry wide reassessment of the value of the V.I., particularly to answer the question, ''What does the V.I. measure.''. Before defining an alternative classification system, it is important to clearly formulate the problem and to realize that both relative and nonrealtive indices suffer from inherent assumptions and limitations, and that the choice of no rating index may be better than the choice of a misleading one.