Engine Oil Classification Explained

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Engine oil is your engine's most important operating fluid, although some might mistakenly claim it is fuel. Indeed, you couldn't start your car without fuel, but the engine would quickly seize up without engine oil.

It is a technologically very complex product used as a lubricant in combustion engines of cars. Besides that, engine oil fulfills the function of cooling, cleaning, sealing, and protecting against corrosion.


This article will shed light on the classification of engine oils, the engine oil labels, and how to read them.

What are the Types of Engine Oils?

There are three types of engine oil if we were to classify them based on production method. Before purchasing any engine oil, it's essential to know what type of oil your car requires. The wrong engine oil might cause fatal damage to your engine since it doesn't properly lubricate the components.

This will increase friction and temperature, and the engine parts will quickly wear. Worst case scenario - you'll seize the engine.

1. Mineral Engine Oil

Mineral oils are produced by processing crude oil. Oil is first freed of water and mechanical impurities. Then fractional distillation divides it into kerosene, gasoline, diesel, and oil fractions. This process divides chemical substances according to their boiling point by heating the temperature at which the individual fractions/components evaporate. Production continues with further distillations, separating oil and fuel from the basic oil fraction. The last distillation gives us the base for engine oil.

All in all, mineral oil can be considered the cheapest of the three types of oils, though it is only used in older cars. It is inefficient in colder temperatures, and they don't like heat, so that both extremes may cause inadequate lubrication. On top of that, they tend to only last up to 5000 km or slightly above 3000 miles.

2. Synthetic Engine Oil

These oils undergo extensive laboratory treatment and are typically created from chemically modified petroleum components. The process breaks down mineral oil into basic molecules - these are very consistent and thus offer superior lubrication. Synthetic oils can also withstand both lower and higher temperatures. The synthesis process that is used to create synthetic oils varies among producers.

Synthetic oils have a higher fluidity at lower temperatures and have a lower evaporation rate. They have better mechanical and chemical properties but are more expensive than mineral oils.

3. Semi-synthetic Engine Oils

It is a mixture of synthetic and mineral oil, while the ratio of synthetic oil is not higher than 30%. They are designed to have the benefits of synthetic oils and the cost of mineral oils. Semi-synthetic oils provide roughly three times the protection compared to mineral oils.

The disadvantage of semi-synthetic oil is that it does not offer superior protection compared to full synthetic oil.

Motor oil

Engine Oil Classification

There are various classification standards and grades when it comes to engine oil. However, there are two main categories representing the engine oil classification and its grades:

  • Viscosity

  • Performance

What is Viscosity?

In a nutshell, viscosity is the engine oil’s resistance to flow. Various engine oils have different viscosities at room temperature and react differently to the temperature change. Engine oil needs to lubricate the engine even in cold temperatures, but it also needs to remain thick enough at high temperatures to prevent friction.

The change in viscosity should be minimal on both sides of the temperature range. The society of Automotive Engineers (SAE) has created grades of engine oils by their viscosity.

SAE Engine Oil Viscosity Classification

The SAE standard uses six winter classes marked with a number and letter "W," and eight summer classes marked the other way around.

Winter classes: 0W, 5W, 10W, 15W, 20W, 25W

Summer classes: W8, W12, W16, W20, W30, W40, W50, W60

Although the SAE viscosity specification is quite important and widely used, it is far from enough to choose the right engine oil for a particular engine. This specification does not tell us anything about other fundamental properties of motor oils, such as oxidation and thermal stability, engine protection against wear and deposit formation, etc.

Performance classifications of motor oils deal with all these parameters and several others. Various institutions create performance classifications. We'll examine API - Association Petroleum Institute, and ACEA - European Automobile Manufacturer's Association, which are the most relevant ones.

API Engine Oil Quality Classification

American Petroleum Institute (API) divides engine oils into two main categories:

  • gasoline engines marked with the letter "S"

  • diesel engines marked with the letter "C"

The second letter that follows an "S" or "C" represents the oil standard - the further the letter in the alphabet, the newer the oil standard (and thus higher). For instance, SN > SL, and so on.

For the diesel engines, though, there may be sub-standards represented by a number after the letters, such as CJ-4.

ACEA Engine Oil Quality Classification

In 1991 The European Automobile Manufacturer's Association replaced the CCMC (Association of Automobile Manufacturers), established in 1972 as a reaction to the fact that API specifications do not fully comply with European engine types structurally different from American ones.

Since 2004, the ACEA classification has divided engine oils into three categories:

  • engine oils for gasoline and diesel engines, labeled "A/B"

  • engine oils compatible with catalytic converters for gasoline and diesel engines, marked "C"

  • engine oils for diesel engines used in trucks, marked "E"

What does 10W-40 mean?

Let's look at the example and translate the alpha-numeric code 10W-40 for the engine oil bit by bit.

  • Since the "W" follows a number, it represents Winter and, thus, how the engine oil reacts to the cold start.

  • The number 10 that precedes the letter "W" represents a parameter that will tell us how the engine oil flows in cold conditions. If the number is smaller, the oil will flow better during cold starts.

  • The number after the dash tells us how the engine oil flows at a normal operating temperature.


You should now have a better overview of the types of engine oils, their classification, and how to read the labels. If you still struggle, let us know in the comment, and we'll try our best to make the information in the article easier to understand.