First principles: No. 2. S.A.E. Numbers.

Abstract

PART One of this series explained the meaning and measurement of viscosity. When any machine builder wishes to recommend a suitable oil for lubrication of any of his products, ho always stipulates the required viscosity. An oil specification will include many necessary properties, but every one will include viscosity limits. Obviously it would not be sensible to give just one viscosity reading, e.g. 110 Redwood seconds at 140°F (or 26.5 centistokes at 140°F). It is usual to give limits within which the viscosity must be, e.g., between 24 and 28 centistokes at 140°F. The limits can be narrow enough to ensure a suitable oil, but wide enough to ensure an oil company being able to meet it with a stock grade.

Citation

(1949), "First principles: No. 2. S.A.E. Numbers.", Industrial Lubrication and Tribology, Vol. 1 No. 2, pp. 14-15. https://doi.org/10.1108/eb051984