An extensive effort is under way to revise the SAE Engine Oil Viscosity Classification System to reflect user needs more realistically. To understand how the present system evolved, the history of the classification is traced, from the original version first published in 1911 to the present 1976 version. Reasons for both high- and low-temperature viscosity grades, multigrading, and footnotes to the viscosity table are discussed, as well as other classification systems which have been discarded over the years.
A critical assessment of the present classification is made from opinions offered at an SAE Open Forum last year. It is pointed out that the system has become quite complicated with four low-temperature and four high-temperature grades, five footnotes, and an appendix. Moreover, the high-temperature viscosity grades are based on an unrealistically low temperature of 98.9°C (210°F) and an unrealistically low-shear (kinematic) viscosity compared with engine operating conditions.
Several suggestions for improving the system are offered, although no one particular approach is advocated. It is concluded that to adequately reflect the effects of oil viscosity on engine performance in the field, the classification system must be revised to include a measure of low-temperature oil fluidity and a high-temperature, high-shear measure of viscosity which correlates with engine performance - a measure which has not yet been developed.