If the used oil sample oil is black in colour, does it need to be changed immediately?
The detergent in the oil functions as a clean up/keep clean agent while the dispersant enables the oil to keep insoluble dirt particles in suspension. If the engine is already dirt clogged, use of good engine oil will result in a lot of the earlier dirt being cleaned off the engine and being kept in suspension. Thus, while the oil is actually performing its job well, a cursory look at a sample of the oil may seem to indicate that the oil has abnormally blackened and needs to be rejected, which is, in reality, a wrong judgment. Two things need to be done if the oil seems very black. Conduct the blotting paper test to see if the oil has the power to keep the dirt continuously in suspension. Secondly, feel the oil to check its viscosity. If it feels abnormally thick, it is possible that the oil has oxidized and, therefore, you are advised to send a sample of the oil to the lab for analysis.
