I. Oxidation of Engine Oil

Oxidation is determined by temperature, time, base oil, and the performance of antioxidant additives. Therefore, oxidation takes a certain amount of time, and in general, the color of engine oil does not darken quickly due to oxidation, let alone turn black quickly.

II.  Vehicle Operating Conditions

Gasoline engines operate at higher temperatures when running at full load, and their cooling systems are designed for continuous operation at full load. If the vehicle starts and stops for various reasons, the engine operates in a "sub-cooled" condition, which is below the designed temperature. During such operation, gasoline is particularly prone to form partially oxidized compounds that migrate into the crankcase, condense into droplets that are insoluble in the oil, and firmly adhere to the metal surfaces when they return to the piston ring area. These deposits undergo thermal conversion into semi-solid or solid varnish-like substances. Additionally, the lubricating oil can undergo nitration and form deposits, a process accelerated by the intrusion of NOX through aeration. If these deposits can be timely flushed away by lubricating oil containing clean dispersant additives and filtered out, it can be very beneficial for engine maintenance.

In the case of diesel engines, during their operation, engine oil darkens rapidly due to the significant amount of soot produced from diesel combustion that enters the crankcase and dissolves into the engine oil. Typically, the higher-grade diesel engine oils tend to darken more quickly and become darker in color when used in the same engine.

Some customers who previously used low-quality engine oil may notice that after switching to high-quality engine oil, the engine oil darkens rapidly. This is because the internal condition of the engine was quite dirty due to the use of inferior oil, and the high-efficiency cleaning and dispersant properties of the high-quality engine oil are taking effect.

Generally, engine oil darkening during use is normal, and the rate at which engine oil darkens depends on the vehicle's condition, driving conditions, engine design, fuel quality, oil quality, and other factors.