THE DARKNESS IN THE OIL USED IN THE ENGINE REFERS TO?

Thick, very dark, or black engine oil usually indicates that your oil has been exposed to dirt or dust contaminants

One of the functions of engine oil is cleaning. A quality engine oil disperses the soot particles
formed in the engine and absorbs them, preventing them from sticking to metal surfaces thanks
to the additives in its content. It also cleans those that adhere to metal surfaces with detergent.
For this reason, engine oil becomes dirty and darkens in color. Therefore, engine oil that looks
dirty means that it is doing its job well.