Mechanical impurities refer to precipitates or colloidal suspensions in lubricating oil that are insoluble in solvents such as gasoline, ethanol, and benzene.

Runningland Talks About Cone Penetration of Lubricating Grease

The content of insoluble matter is reflected in the ability of the lubricating oil to hold pollutants, mainly composed of oxidation products and wear metal particles.