What turns engine oil black?

A common way to check your oil is dipping the dipstick in the engine oil fill port and check the color from its tip. As oil ages, it loses its original color, amber, meaning it has lost its protective properties and needs to be changed. The more you drive, the more engine oil is used up. However, every time engine oil turns black does not mean it needs to be changed.

Why is engine oil dark after use?

Exposure to heat

engine oil turning black

Once you drive your vehicle anywhere, the engine reaches normal operating temperatures (90°C – 104°C), heating the engine oil. Once you park your vehicle, it cools down, cooling the motor oil. This continuous process of exposing engine oil to heat cycles causes it to darken. The more driving trips you make, the more heat cycles your motor oil experiences.

Engine oil contains certain additives, chemical compounds, to increase the oil’s performance. These additives decompose at different rates when exposed to heat. As a result, your oil is likely to turn black.


Normal oxidation darkens oil too. Oxidation of engine oil occurs when air mixes with oil in the internal combustion engine, causing a chemical breakdown. High temperatures increase the rate of oxidation, leading to engine oil losing its original properties.

Long periods of using the same motor oil, fluctuating or high engine operating temperatures, and an increase in viscosity and insoluble compounds cause the oxidation of motor oil. Oxidation is similar to when you cut an apple and expose it to oxygen, causing it to turn brown.


Diesel and gasoline engines are susceptible to soot since it is a byproduct of incomplete combustion. Usually, the oil filter traps soot particles. At times, you may find these soot particles have combined to form larger wear-causing contaminants, leading to the formation of sludge if the oil filter does not catch them.

Soot particles are less than a micron in size. The oil filter may sometimes fail to filter all of them out, leading to black oil. Some modern cars contain finer filtration systems that trap dissolved additives in motor oil.

Why does synthetic oil turn black faster?

synthetic oil

It is normal for motor oil, especially in diesel engines, to quickly turn black, meaning the oil is effectively performing its role by keeping byproducts from the combustion process in suspension.  Synthetic oil is known to go black faster than regular oil.

Conventional oil, also referred to as regular oil, is distilled and refined from crude oil. On the other hand, synthetic oil is made from various chemical compounds to produce better performing oil that meets the specific needs of modern vehicles. As such, synthetic oil contains more additives which help in making the oil more efficient. These additives deteriorate when exposed to heat, and some of them turn to soot, causing the oil to turn black faster.

Is dark engine oil bad?

Often, people assume that oil that has turned black is too saturated or too contaminated to protect the engine, meaning it needs a change. This is not necessarily true. Black oil is an indication of engine health. Furthermore, discoloration of engine oil is a normal process caused by natural byproducts of soot particles and heat. These byproducts rarely cause wear and damage to your engine.

Is dark engine oil bad?

Synthetic oil breaks down longer as compared to conventional oil, hence replacing it is less frequent than if you use conventional oil. Most mechanics recommend changing synthetic oil after 7500 miles, though some of them last up to 10,000-15,000 miles.

The only sure way to find out if your engine oil is bad and requires changing is by conducting a thorough oil analysis. Several companies offer oil analysis services where they use an oil sample to understand the condition of the oil, percentage of contaminants present, and fuel dilution, among many others.

Another way of determining if your engine needs an oil change is by checking your owner’s manual or the instruction manual from the motor oil. These manuals consist of specific guidelines on how to maintain and operate your vehicle. Always remember it is crucial to adhere to these guidelines strictly.


As discussed above, several factors turn engine oil black. When it turns black, it does not necessarily mean it is time for an oil change. It is up to car owners to regularly check the color of the engine oil. If something does not tally up, it is best to consult your owner’s manual, mechanic, or dealership for further analysis.

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