Why does engine oil become dark brown / black after use for a long time?

Engine oil naturally darkens over time due to several factors, such as heat cycles during normal operation, which cause the oil to heat up and then cool down while the car sits.

Some additives in the oil are more susceptible to darkening in the presence of heat, and normal oxidation can also contribute to oil darkening.

Additionally, the oil’s viscosity changes over time and it can become contaminated with dust and dirt, resulting in a muddy and dark appearance.

Thick, very dark, or black engine oil often indicates that the oil has been exposed to dirt or dust contaminants that cause soot.

However, the color of the oil does not necessarily reflect its efficiency, and it’s not possible to judge the efficiency of oil based on color alone.

Neglecting to change the oil regularly can cause the lubricating oil to break down, become contaminated, and stop flowing, resulting in a thick and wasteful substance