Most drivers choose their engine oil by viscosity, 5W-30, 0W-20 or 5W-40. However, viscosity only describes how the oil behaves at different temperatures and says nothing about its performance properties: whether the oil can handle the load of a specific engine, whether it is compatible with a catalytic converter or particulate filter, and whether it suits the required service interval.

Why approvals appeared

As engines evolved, turbochargers, direct fuel injection, catalytic converters, particulate filters and longer oil-change intervals all appeared. A single SAE classification, which describes only viscosity, was no longer enough.

Each of these technologies places its own demands on the oil. Turbocharging and direct injection raise operating temperature and the load on the oil film. Particulate filters and catalytic converters are sensitive to sulphated ash, phosphorus and sulphur content in the oil, these elements can clog the filter pores and reduce the efficiency of the converter. Longer service intervals require an additive package that is more oxidation-resistant and more durable. This is exactly why SAE viscosity alone became insufficient, and additional classification systems appeared.

History of oil approvals

API, ACEA and OEM approvals

API defines the minimum level of performance properties. ACEA sets European requirements, which are usually stricter than the American ones and account for the specifics of European engines and emissions rules. OEM approvals confirm compliance with the specific requirements of the car manufacturer and are the most precise selection criterion.

How the API code works, and how it has changed

The API classification consists of two letters. The first letter indicates the engine type: S (Spark) means petrol engines, C (Compression) means diesel engines. The second letter indicates the category level: the further it is from the start of the alphabet, the newer and more demanding the standard. The letters I, K and O are not used in this sequence to avoid confusion with other labelling systems. The first API classification was published in 1947, and it has been regularly updated ever since to keep pace with engine development.

Diesel engines have a separate sequence of categories with the letter C.

API designs its categories so that a newer category is generally backward compatible with older requirements for the same engine type. However, this does not remove the need to check the exact category specified by the car manufacturer, since in some cases (for example, for older engines, or for narrowly specialised categories such as FA-4) a formula that is too new or incompatible may be undesirable.

Besides the letter category, the API "donut" symbol may carry a separate Resource Conserving (RC) mark. It means the oil has passed an additional fuel-economy test against a reference oil, and it always appears together with the main category, not in place of it. The absence of the RC mark does not mean the oil is worse: it simply means the manufacturer did not claim or confirm that particular separate test.

The ILSAC standard and its link to API

ILSAC (International Lubricant Standardization and Approval Committee) is a committee established in 1992 by American and Japanese car manufacturers. ILSAC standards are built on API categories but add stricter fuel-economy requirements and are labelled GF (Gasoline Fueled). ILSAC standards apply only to petrol engines and only to certain viscosities.

What the ACEA categories mean

The ACEA (European Automobile Manufacturers' Association) classification divides oils into several letter categories by engine type and compatibility with exhaust after-treatment systems. Unlike API, ACEA categories are not simply extended over time, they are periodically fully revised: obsolete categories are withdrawn and new ones are introduced in their place.

The C categories appeared later than the others precisely because of the spread of particulate filters and catalytic converters: oils with higher ash, sulphur and phosphorus content must not reach the exhaust system in such engines, since this can prematurely damage costly components.

How to find the right approval for your car

The exact approval for your car is listed in the owner's manual. This document, rather than general recommendations found online, is the main source when choosing an oil.

Common misconceptions

All 5W-30 oils are the same.

No. Oils of the same viscosity can have different additive packages and different approvals, which means different performance properties and compatibility with specific engines.

If the viscosity matches, the oil is suitable.

No. Viscosity only determines how the oil flows at different temperatures. The manufacturer's approval must also be met, since it governs the chemical composition and compatibility with a specific engine.

An oil with a higher approval is always better.

No. A newer or higher API or ACEA category is not always suitable for a specific engine. What matters is an exact match to the approval specified by the car manufacturer, not a higher category by itself.

Only genuine dealer oil carries the right approval.

Not quite. Official OEM approvals are issued by car manufacturers to third-party oil producers that have passed the relevant tests and paid for the certification process, which can be quite expensive. This is exactly why not every good-quality product carries an official approval: for the manufacturer, this represents a real cost that is not always justified for a given market or sales volume.

Alongside oils with an official approval, the market also has oils that a manufacturer states meet the requirements of a given approval, or recommends for it, without formal certification from the car manufacturer. This is a common and entirely workable practice: such an oil can be used too, and its quality is defined by its specifications and the manufacturer's reputation, not solely by the presence of certification.

Frequently asked questions

Can I use an oil with a higher approval than the manufacturer requires?

Generally, a newer API or ACEA category for the same engine type is backward compatible, but an exact match to the manufacturer's OEM approval remains the priority selection criterion.

What should I do if no oil with the exact required approval is available?

You can choose an oil that the manufacturer states meets the requirements of the required approval, or recommends for it, even without formal certification from the car manufacturer. It is enough to check the oil's technical data sheet and confirm the viscosity and required category are correctly listed.

Does the approval differ for petrol and diesel engines?

Generally yes: with API this is reflected in the first letter (S for petrol, C for diesel engines), and with ACEA in different letter categories (A, B, C, E). At the same time, many oils carry two approvals at once and suit both engine types simultaneously, for example the ACEA A3/B4 category inherently combines requirements for both petrol and diesel engines.

Do I need to change the oil approval when switching to a different brand?

No, when switching brands it is important to keep the same OEM approval, viscosity and API and ACEA categories specified by the car manufacturer, rather than basing the choice on the brand of the previous oil.

Conclusion

When choosing an oil, look first at the car manufacturer's OEM approval, then at API or ACEA requirements, and only after that at SAE viscosity. This order guarantees that the oil will not just have the right flow behaviour, but will also fully protect the engine, remain compatible with the exhaust after-treatment systems, and support the required service interval.