What is the composition of motor oil?

Whether mineral, synthetic or semi-synthetic, motor oil consists of a base oil and additives. Let’s take a closer look at what goes into this essential lubricant, and the role of the main additives used.

Composition of mineral, synthetic and semi-synthetic oil

There are three kinds of motor oils: mineral oil, synthetic oil, and semi-synthetic oil. Each of these motor oils has a different composition: the base is either refined crude petroleum, or polymers manufactured in the laboratory.

Mineral oil: composition of a natural oil

Mineral motor oil is made from crude petroleum. Once extracted and transported to a refinery, the crude petroleum undergoes several refining processes to remove as many impurities as possible. This oil mainly consists of hydrocarbons (oxygenated or non-oxygenated), but it may also contain traces of compounds such as sulphur or nitrogen. After processing, additives are added to the so-called mineral motor oil to improve its performance.

How do you make synthetic oil?

Synthetic oil can be created chemically in the laboratory, but it can also come from refined petroleum. Refining synthetic oil is a more complex process than for mineral oil: it involves modifying the structure of the hydrocarbon molecules. The processes for obtaining the oil ensure that only the best molecules are retained. Many additives are also added. Given its composition, so-called synthetic motor oil is considered a high-end oil that is purer and delivers a higher performance than mineral oil.

What is semi-synthetic oil made of?

Semi-synthetic oil is a blend of synthetic oil and mineral oil. The proportions in the mixture are not equal: semi-synthetic oil contains at the most 30% synthetic oil. It is interesting to know that oil with as little as 1% synthetic oil is also called “semi-synthetic oil”.

What is the purpose of the additives used in motor oil?

To ensure the quality, performance and lifespan of motor oils, additives are added to both mineral oils and synthetic oils.

Type of additive	Role of additive
Oxidation inhibitor additives	Over time, motor oil oxidises. It lubricates engine parts less effectively. That’s why motor oil must be changed regularly. However, the oxidation inhibitor additive slows this process, improving the motor oil’s lifespan.
Detergent additives	The main property of a detergent additive is to remove the deposits and microparticles that form on surfaces when the vehicle is used.
Dispersant additives	Dispersant additives keep all the deposits and impurities that form when the engine is running in suspension, thus avoiding deposit build up on engine parts. The deposits are then carried to the oil filter.
Antifoam additives	The use of detergent additives can generate foam on the surface of the oil. This prevents the oil from lubricating engine parts properly. This additive is used to prevent this occurance.
Anti-wear additives	Also called friction modifier additives, anti-wear additives add a layer of solid oil that lubricates the engine as soon as the vehicle starts up, thus avoiding friction between parts. This additive is mostly used in mineral oils.
Corrosion inhibitor additives	An engine is made of metal parts that are sensitive to rust. Corrosion inhibitor additives protect parts from rust.
Viscosity index modifiers	These additives reduce the difference in viscosity between cold oil and hot oil.
Antifreeze additives	These additives help adapt the various motor oils to their environment by modifying the oil’s hardening temperature. They increase the fluidity of cold oil.