Why is the drained engine oil black?

1 Answers
MacJoshua
07/30/25 1:30am
Why is the drained engine oil black? The reason is that the oil change interval is too long. Engine oil primarily lubricates various components inside the engine, especially playing a crucial role in lubricating pistons and cylinder walls. The combustion byproducts in this process form dark-colored viscous substances. Engine oil, motor oil, or engine lubricant is a substance enhanced with additives in base oil, mainly containing anti-wear additives, detergents, dispersants, and multi-grade oil viscosity index improvers. The main functions of engine oil are to reduce friction and wear on moving parts, remove sludge (one function of dispersants) and varnish (function of detergents) from the engine, neutralize acids produced by fuel combustion and lubricant oxidation (function of detergents), improve piston ring sealing, and cool the engine by carrying away heat from moving components.
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