Why does engine oil turn black after a period of time?
2 Answers
It is normal for engine oil to turn black after a period of time because the oil is directly added to the engine. Long-term engine operation produces carbon deposits, and the role of engine oil is to clean. Since the oil contains detergents and dispersants, it can clean and dissolve carbon deposits and oxides inside the engine. New oil can quickly develop black substances, causing it to turn black. Normally, engine oil should be semi-transparent, but some people may encounter situations where new oil turns white after driving for a while, accompanied by foam. This is caused by the presence of water in the oil. Therefore, if the used oil appears milky white, it may be due to a leaking radiator or external water mixing into the lubrication system. In such cases, the oil should be drained, and the vehicle should be inspected. Continuing to use it could jeopardize driving safety.
As someone who frequently tinkers with cars, I find it quite normal for engine oil to turn black. This is primarily because the oil is performing its cleaning duties, absorbing dust, carbon particles, and metal shavings generated during engine operation. These contaminants are byproducts of the combustion process and are suspended by the detergents and dispersants in the oil, preventing them from clogging oil passages or causing wear. Additionally, engine oil operates under high temperatures and pressures, and over time, it oxidizes and degrades, gradually darkening in color. This is actually a sign that the oil is functioning properly and healthily. If the oil doesn't darken at all, it might indicate that it's not performing its cleaning role. However, if the oil suddenly turns unusually black or appears milky white in a short period, it could suggest a coolant leak or incomplete combustion, requiring immediate inspection. I usually follow the manual's recommendation to change the oil every 5,000 kilometers and opt for high-quality brands to protect the engine and avoid long-term accumulation leading to more severe engine issues.
