Is the American Petroleum Institute (API) rating useful for determining a motor oil’s viscosity?

The American Petroleum Institute (API) rating is primarily designed to indicate the quality and performance level of motor oils rather than their viscosity. The viscosity of an oil is typically described by a separate grading system, such as the Society of Automotive Engineers (SAE) viscosity grades, which designate how thick or thin the oil is at specific temperatures.

While the API rating provides crucial information regarding the oil's ability to perform under certain conditions—like wear protection and thermal stability—it does not directly communicate the oil's viscosity characteristics. Therefore, saying that the API rating is useful in determining a motor oil's viscosity is not accurate. Instead, one would refer to the SAE classification to understand the viscosity aspect of the oil.

Thus, the right understanding is that any determination of viscosity must rely on other classifications and metrics outside of the API rating.