Why Does Engine Oil Turn Black?
2 Answers
Engine oil turns black because there is some carbon deposits and sludge inside the engine. Engine oil has a cleaning function, so after being used for a period of time, the oil will turn black as it cleans away the dirt in the engine. It is normal for engine oil to darken after some use. The presence of some carbon deposits and sludge in the engine is also normal, and the oil can clean away part of these deposits. Engine oil is composed of base oil and additives. During normal engine operation, the oil forms a protective film on the surfaces of various engine components, preventing direct contact and friction between them.
It's perfectly normal to find engine oil turning black, mainly because it absorbs combustion residues like soot, ash, and acidic substances while working inside the engine. The high operating temperatures and piston movements generate various fine particles. The detergent additives in the oil act like a broom, encapsulating all these contaminants to prevent clogging, which gradually darkens the oil. When I first bought my car, I always worried this meant something was wrong, but later I understood it's actually a good sign—it shows the oil is protecting the engine. As long as you change it every 5,000 to 8,000 kilometers as recommended in the manual, the color change is healthy. Additionally, certain driving habits, such as frequent short trips or stop-and-go city driving, can cause the oil to darken faster. So, I recommend regularly checking it with the dipstick—uniform color is what matters.
