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FAQ Number 65
What does “viscosity grade” and “GL rating” mean?
A:  The Society of Automotive Engineers (SAE) has designated a grading system that classifies gear oils by their viscosity. Normally, oils are graded at one temperature (usually 100° C). Winter oils are also graded at -18° C. When the oil is graded at the lower temperature it is given the suffix “W” (i.e. 80W). Many oils are formulated to meet the requirements of more than one classification. Multi-grade oils are designated by two grade numbers such as 80W90. This number indicates the oils lowest and highest grading. While viscosity is important, the American Petroleum Institute (API) classification is critical in determining proper gear oil. The API has classified gear lubes (GL) into several categories based on applications, properties and additives. The classifications are GL-1, GL-2, GL-3, GL-4, GL-5 and GL-5* (GL-6 has been replaced by GL-5*). Vehicle and equipment manufacturers have specific gear oil requirements and viscosity grades. This information should be specified on the gear case or in the service manual.

Name Type Date
Created On
November 10, 2003
Created By
CRC Industries
Asked By