Why does motor oil darken and appear dirty?

One of the functions of motor oil is cleaning. A quality motor oil uses its dispersant and detergent additives to disperse and absorb soot particles that form in the engine, preventing them from adhering to metal surfaces.

It also cleans deposits from metal surfaces using its detergent properties. This is why motor oil appears dirty over time. Therefore, motor oil that looks dirty is actually doing its job well.