What are the main viscosity classes of motor oils according to SAE?

The generally accepted viscosity classification system for motor oils is the SAE (Society of Automotive Engineers) system.

The SAE system divides oils into two main groups:

• summer (with a numeric index, for example, SAE 30, SAE 40);
• winter (with a W index and a numeric index, for example, SAE 0W, 5W, 10W, 15W, 20W).

The most common today are all-season oils that combine the properties of summer and winter oils. They have a combined marking, for example, SAE 5W-30 or SAE 10W-40.

The first digit (with the W index) indicates low–temperature properties (fluidity at cold start), and the second digit indicates high-temperature properties (viscosity at engine operating temperature).