Motor oil: viscosity is not the whole specification

Useful to know

In motor oil, distinguish viscosity class from operating specification. In the API manual, viscosity is explained by flow behavior at certain temperatures, and the level of performance is a separate part of the designation. [1]

Therefore, two oils with the same large designation as 5W-30 should not be automatically assumed to be the same in all respects. A specific engine may also require other characteristics.

Practically made two separate checks in the vehicle manual: allowable viscosity and required specification or approval. Then compare the exact product with them. Don't replace the choice with a general "thicker is better" rule.

This API document is a 2010 edition. We use it to distinguish concepts, not as an up-to-date list of categories in place in 2026. For real service, follow the manufacturer's current requirements.