Is darkening of the engine oil is a normal phenomenon?

     Yes, this is absolutely normal, because the quality oil should neutralize the products of decomposition in the engine after combustion of fuel. After all, it is the products of decay, which may be sulfur, soot, tar, varnish, various acids depending on the type of fuel, significantly affect the color of motor oil during operation. It is necessary to worry when the color does not change during long-term operation. This may be a sign of a low alkaline amount of oil which is commonly referred to as washing the number of people. In the certificate or in the catalog for oil it should be indicated. It is desirable that for petrol and gas engines it was from 6.5 to 11 mgKON / g and 7-14 for diesel engines. The darkening of the oil within reasonable limits should not be feared. A similar situation requires washing dirty hands, using an alkaline solution (for example, soap), we see contaminated water, but clean hands. Similarly, with oil in the engine, it will darken, but the engine will remain clean.