Why does engine oil eventually turn black

Dark engine oil on the dipstick does not always mean it is bad, as car owners often think when checking its level in the engine. To find out if it is bad or the norm, you need to do a simple test.

Gradual aging and darkening is a natural process. Therefore, the main task of the motorist is to figure out whether the darkening is caused by positive or negative factors.

Causes of blackening should be considered in each case separately. What engine are we talking about (gasoline or diesel), how quickly it turned black (after replacement or passed a few thousand miles)? The reasons can be very different: dirty engine condition, long mileage without changing the car, switching to an oil with a lot of cleaning additives, pouring not heat-resistant oil (not designed to work in this case), overheating the engine.

Why is it normal if the oil darkens gradually?

The blackening process is due to the fact that among other functions of the lubricating fluid there is a cleaning one. During the engine operation, there is combustion of fuel with the appearance of complex chemical compounds, the composition of which depends largely on the type and quality of fuel, but they are always harmful to the engine. These residues are located directly in the engine (on the walls of cylinders, pistons, rings, and so on).

Oil, by removing deposits from the surface of the parts, keeps them in their composition, not allowing them to be deposited in the engine. That is, combustion residue passes into the composition of the engine oil, and this leads to its natural darkening.

When is rapid darkening of engine oil considered normal?

Diesel combustion produces darker exhaust fumes, similarly producing blacker deposits in the engine. This means that the oil gets darker (twice as fast). As for engines running on gas (methane, propane, butane), the situation is reversed. Due to the chemical composition, there are less dirt deposits in gas (like in oil products), so they produce much cleaner exhaust gases. Accordingly, the engine oil in this case will be cleaner.

It is also worth noting the fact that the oil in the engine will darken faster if it has recently been overhauled. In this case, the darkening is caused by a large number of metal chips, resulting from the interaction of rubbing pairs. After the «overhaul» it is recommended to change the oil before the period prescribed by the regulations.

You should not worry if it became black after using oil with a high quantity of cleaning additives (high ash content, TAN). But it is worth observing its condition. If it looks homogeneously viscous and homogeneously dirty — then the cleaning process is going well. But it will be necessary to change it much earlier than the standard term!

After how many kilometers should the oil get darker?

Oil darkens naturally if it happened by two to three thousand kilometers of mileage. And when there are no additional symptoms of engine malfunction (it runs steadily, does not «eat» oil), then there is no reason to worry.

If the oil turns dark quickly (almost immediately after changing it or at short mileage, after 800 — 1000 km run) — this is the reason to perform diagnostics and search for the causes, which will be discussed below.

Black engine oil? Why does your engine oil go BLACK and should change it? Engine Oil Explained

Negative causes of blackened oil

Depending on the rate of darkening and the accompanying symptoms, dark brown or black engine oil may become so for a number of reasons and malfunctions due to which it quickly loses its properties. Some of them can be identified by a drop test, but to determine some you will have to check the condition of the engine.

1. The oil turned out to be of poor quality or not designed for the existing operating conditions of the car.
2. Combustion products got in.
3. Engine overheating.
4. Bad quality of fuel.
5. The crankcase ventilation system is malfunctioning or engine wear.

Improper oil

Mineral oils and oils with high ash content and low base number (TBN) darken most rapidly. The ash index is a measure of the number of unburnable elements in the ash content and the detergent ability of an oil.

The higher the TAN but the lower the TBN, the more detergent capacity of the lubricant and thus the darker the oil will be in service. These values are selected and recommended by the car manufacturer for a specific engine. Therefore, if the oil has not been selected for the specific engine, it will lose its properties faster. The same pattern is true when the manual provides for the use of synthetic oil with a high temperature operating threshold instead of which a mineral or semi-synthetic one is used.

Engine malfunction

Source image: parts.olathetoyota

The main reason for the fuel combustion products getting into the oil is the cylinder head gasket rupture or damage of the scraper rings. Such malfunctions will necessarily be accompanied by additional symptoms. For example, if the cylinder head gasket is damaged, the fluid will be bubbling in the expansion tank, and if there are problems with the rings, increased oil flow will be observed.

Also, gasoline can get into the oil, it liquefies it and changes its color. You can determine such a problem by the smell of oil on the dipstick.

Another reason for the blackening of engine oil is the contact of crankcase gases with oil. This happens when the crankcase gas ventilation system is defective. Because of this, it oxidizes faster, loses its operational properties and turns dark. And the more gases get into the engine, the faster this process happens.

The oil has been subjected to severe overheating

Under severe operating conditions, with the engine running at high ambient temperatures, poor ventilation, and at high speeds the oil literally fries and turns black. That is why you should try not to run the engine constantly at critical loads, or choose an oil with the proper performance characteristics (viscosity, tolerances and additives).

The fuel quality used is lower than that prescribed by the manufacturer

As the main cause of oil darkening is the products of petrol/solar oil combustion, the fuel composition is very important. The cleaner the fuel (made to a higher standard, with no additional impurities), the less dark will be the combustion products, and thus the less dark will be the engine oil. Therefore, try to pour good fuel into the tank, it will not only increase engine power but also will save it from excessive wear and tear, as well as increase the engine oil life.