Engine Oil Types and Differences: How to Choose the Right One

The right engine oil is the one your car’s manufacturer specifies, not the one with the most impressive label. Two things decide it: the viscosity grade (for example 5W-30) and the performance specification or approval (for example ACEA C3 or a manufacturer code). Whether the oil is mineral, semi-synthetic or fully synthetic is a third question, and in modern engines it is usually answered for you by the first two.

This guide explains what each part of the label means, how the oil types differ, and how to avoid the most common mistakes. Last updated: October 2026.

What engine oil does

Engine oil lubricates moving parts so metal does not rub against metal. It also carries heat away from hot areas such as the pistons, holds combustion by-products in suspension until the filter and the next oil change remove them, and protects internal surfaces against corrosion. Additives (detergents, anti-wear agents, viscosity modifiers) are what make an oil more than «slippery liquid», and they are the reason two oils with the same viscosity can behave very differently.

Step one: find what your engine requires

Open the owner’s manual, or the service section if there is a separate booklet. Look for three pieces of information:

• Viscosity grade, such as 0W-20, 5W-30 or 5W-40.
• Specification, such as an ACEA category or an API service category.
• Manufacturer approval, if listed (a code defined by the car maker, which some oils carry on the label).

If the manual lists several options, the engine was designed for any of them. If it lists an approval code, the oil you buy should state that same code on the label. A category that looks «better» is not automatically a valid substitute.

How to read a viscosity grade such as 5W-30

Viscosity is how easily the oil flows. The grade follows the SAE system and has two parts:

• The number before the «W» (W stands for winter) describes cold behaviour. A lower number means the oil flows more easily at low temperatures, which matters at start-up, when most engine wear occurs.
• The number after the dash describes thickness at normal operating temperature. A higher number means a thicker oil film when the engine is hot.

Thicker is not «stronger» and thinner is not «weaker». Modern engines have tight tolerances and narrow oil passages designed around a specific viscosity, so using a grade that differs from the specified one can reduce protection or increase fuel consumption. Only change grade if the manual allows alternatives for your climate or driving conditions.

Mineral, semi-synthetic and synthetic oil

The type describes the base oil that makes up most of the product.

Type	What it is	Typical use	Main limitation
Mineral	Refined from crude oil	Older, simpler engines whose manual still allows it	Less resistant to heat and ageing; fewer modern engines allow it
Semi-synthetic	A blend of mineral and synthetic base oils	Engines that specify it, or a compromise where the manual allows	The proportion of synthetic varies by brand, so the name alone tells you little
Fully synthetic	Base oils produced chemically to tighter, more uniform properties	Most modern engines, turbocharged and direct-injection engines	Higher price; still must match the required specification

Synthetic oils generally resist heat and oxidation better and flow better when cold, which is why many manufacturers require them. But «synthetic» on the front of the bottle does not replace the viscosity and specification requirements above.

Specifications: ACEA and API in plain language

Specifications are performance standards an oil has been tested against. Two systems appear most often on labels.

ACEA (common on oils sold in Europe)

ACEA is the association of European car manufacturers, and its oil sequences are updated every few years. The codes are read as a letter plus a number, followed by a year suffix (for example, «C3-21» or «-23») that shows which edition of the standard the claim refers to.

• A/B categories cover petrol and light-duty diesel engines. A3/B4 is a higher-protection category; A5/B5 is aimed at lower-viscosity, fuel-economy oils. The older A3/B3 category was removed in the 2021 light-duty update, and A7/B7 was added.
• C categories are «catalyst-compatible» oils with limited ash, phosphorus and sulphur (low or mid «SAPS»), designed to protect exhaust after-treatment such as diesel particulate filters. Many modern diesel cars with a DPF require a C category oil.
• E categories are for heavy-duty diesel engines and are not relevant for most cars.

An important point: a higher number or a newer category is not a universal upgrade. For example, C3 and C5 have different minimum high-temperature viscosity requirements, so one cannot simply replace the other unless your manufacturer allows it. An ACEA claim tells you which performance level the oil meets; it does not tell you whether it is approved for your specific engine.

API (common on oils sold worldwide)

The American Petroleum Institute uses an «S» prefix for petrol engines (for example SN or SP, with later letters being newer) and a «C» prefix for diesel. Check your manual for the minimum category and the current one on the label, as the standard has been revised several times.

Quick decision guide

Your situation	What to check first
Diesel car with a particulate filter	Whether the manual requires a C-category (low-SAPS) oil; do not substitute an A/B oil
Modern turbocharged petrol engine	Viscosity grade and any manufacturer approval code listed in the manual
Older car, high mileage	Manual specification first; «high-mileage» oils are only appropriate if they also meet that specification
Car under warranty	Use oil that meets the manual’s requirements and keep receipts for service history
Cold winters or very hot climate	Whether the manual lists alternative viscosity grades for those temperatures

Common questions

Can I mix different engine oils?

In an emergency, topping up with a different oil is better than driving with a low level, and modern oils are formulated to be compatible with each other. It does dilute the properties of the better oil, so for a long-term fix, do a full change with the correct oil at the next opportunity.

Is synthetic oil worth the extra cost?

If your manual requires synthetic or a specification that only synthetic oils meet, it is not optional. If your manual allows several types, synthetic generally offers better protection under heat and cold, but whether the extra cost pays off depends on how you drive, and the manual’s change interval should still be your guide.

How often should I change the oil?

Follow the interval in your manual, expressed in distance or time, whichever comes first. Short trips, towing, dusty roads or heavy stop-and-go driving put the oil under more stress, and many manuals list a shorter «severe use» interval for those conditions.

Does the brand matter?

The brand matters less than whether the oil carries the viscosity, specification and approval your engine needs. If you are comparing two well-known brands, see our separate comparison: differences between Motul and Castrol engine oils.

Mistakes that damage engines

• Choosing by price or bottle claims alone instead of by specification.
• Using a thicker oil «to protect an old engine» without the manual’s backing; it can reduce oil flow at start-up.
• Overfilling. Too much oil can cause foaming and pressure problems. Read what happens with too much oil in the engine.
• Ignoring the oil warning light. If it comes on, stop safely and check the level; see what the oil warning light means.
• Confusing engine oil with gearbox oil. They are different fluids with different change intervals; see when to change gearbox oil.

Signs the oil may be causing or revealing a problem

If the oil level drops between changes, check whether the exhaust shows blue-grey smoke, which can indicate oil being burned (causes of white or blue smoke). A creamy, milky residue under the oil filler cap or oil appearing in the coolant (oil in the antifreeze expansion tank) needs a mechanic’s attention rather than a different oil.

Summary

Start with the manual: viscosity, specification and approval. Then choose between the oils that meet them, using type (mineral, semi-synthetic, synthetic), climate and driving conditions to decide. If the manual and the bottle disagree, the manual wins.

Further reading: a third-party explanation of how ACEA categories are structured is available at oilspecifications.org. Verify the latest ACEA edition with the oil manufacturer’s product data sheet before buying.

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