Why does engine oil turn black?

There are several questions asked about engine oil. One of the frequent questions asked is “why does engine oil turn black?” and when the oil darkens, does it mean it is time to change it? When new oil is poured into the engine during service or top-ups, its colour is gold. However, when a dipstick is inserted in the compartment for an oil check, the colour is usually darker than the new oil. This article will outline the reasons why the oil turns black.

Engine oil plays three vital roles in any engine regardless of its model or type. The first obvious and widely known role is to reduce friction via lubrication. Engine oil helps in reducing friction within the engine between the moving parts to prevent lock-up resulting in engine failure. Engine oil also acts as a heat exchanging fluid in the compartment. Due to the combustion, a large amount of heat is produced in the engine which needs to be controlled. Engine oil is used to adsorb the heat from the engine and transfer it to the sump where it is cooled by the continuous airflow. The third function of the engine oil is to clean the compartment.

Engine oils have additives that act as detergents for clean up. In simpler terms, assume a washing machine analogy. If dirty clothes are put in the machine along with some detergent to clean them, the clothes would be cleaned but the wastewater coming as effluent will be dirty. This is the case with the engine, the compartment is cleaned and as a result engine oil turns black. Oil usually suspends particles such as carbon, dirt and other contaminants which are harmful to the engine which eventually makes it darker. You should be worried if the colour is still gold (or the same as new oil) after some time (the detergent is not functioning as it should).

One major contaminant is soot. Soot is a byproduct of incomplete combustion in the engine. Generally, people tend to believe that soot is only associated with diesel engines. However, it should be clear that soot can also be produced from petrol engines, especially the new gasoline-direct-injection engines. Usually, soot particles are small in size, generally less than a micron and therefore, they do not cause engine wear.

In some cases, soot particles may agglomerate forming larger particles that can cause wear. However, they are mostly captured in the oil filter. Often people use bypass filtration systems to capture particles as small as two microns but still notice that the oil turns black. This is because the soot particle can even be smaller than two microns and therefore, elude down the filtration system. If a filter smaller than two microns is put in place, it will start capturing the additives and therefore not practical.

Despite the above discussions, a question would still be raised as to why the oil darkens quicker in some vehicles compared to others. Oil darkening highly depends on the number of kilometres (or hours) the equipment has been used since its manufacture. As the vehicle or the equipment ages, carbon contents starts to build up in the system. In cases of new machines, the oil would retain the golden colour for a few hundred kilometres or hours since the carbon has not started to build up. As the same machine ages, the carbon content starts building up and as a result, darkening begins. This should however not be of concern as it is just a byproduct of using the machine as long as you are using the right and high-quality oil, changing it and replacing the oil filter regularly.

About the Author:

Rajan Hirani is ILD/Sinopec’s Technical Support Officer. He is responsible for the chemical analysis of lubricants, delivering subsequent recommendations on the most effective lubricants and application technologies to customers. Rajan is a PhD Research Scholar in Chemical Engineering. He holds a Bachelors degree in Chemical Engineering (First Class Honours) and an Advanced Diploma in Science and Engineering.