What 5W-30 means and why it matters
The number pair on the canister is the SAE viscosity grade — in practice, how thick the oil is at two different temperatures. The figure before the W (for winter) describes cold behaviour: 0W flows more easily than 5W, and 5W more easily than 10W. The sooner oil reaches the bearings in the first seconds after start-up, the lower the wear — and most engine wear happens exactly then.
The second figure, 30 or 40, describes viscosity at 100 °C, that is at operating temperature. Thicker does not automatically mean better protection: modern engines have narrow galleries, calibrated oil pumps and variable valve timing systems that depend on a precise flow rate. A 5W-40 poured into an engine designed for 0W-20 can slow the hydraulic timing actuators and increase fuel consumption.
ACEA, API and carmaker approvals
Viscosity alone is not enough. The specification describes the oil's chemistry: the additive package, sulphated ash content, oxidation resistance and compatibility with exhaust after-treatment. Europe uses the ACEA classification: A/B classes are for petrol and diesel engines without a particulate filter, C classes are low SAPS — reduced sulphated ash, phosphorus and sulphur — for cars with a particulate filter (DPF or GPF) and catalytic converter, while E classes are for trucks.
The American API classification uses S for petrol (SN and SP being recent) and C for diesel. Above both sit the carmaker approvals: VW 504.00 and 507.00, MB 229.51, BMW Longlife-04, PSA B71 2290, Renault RN17 or GM dexos. These are the manufacturers' own tests, stricter than the ACEA minimum, and if the manual demands a specific approval it is mandatory, not a marketing suggestion.
How to read a canister label
Any serious canister lists, in order: the SAE viscosity grade, the ACEA and API classes, then the approvals. The important distinction is between the wording “meets the requirements of” and an actual approval officially listed by the carmaker. For a car still under warranty, look for the explicit approval.
Check the quantity too: an oil change typically takes between 3.5 and 6.5 litres depending on the engine, and a 5-litre canister rarely matches the requirement exactly. One spare litre kept in the boot covers top-ups between services.
What happens if you use the wrong oil
The most expensive mistake is a high-ash oil (ACEA A3/B4) in a diesel with a particulate filter: ash builds up in the filter, which can no longer regenerate fully and has to be cleaned or replaced. At the other end, an oil that is too thin in an older engine with wide clearances raises oil consumption and running noise.
The wrong oil will not destroy an engine in a day, but it dramatically shortens the time to the first problem and can void the warranty. If you have already used the wrong oil, the simplest fix is a full change, with a new filter, at the earliest opportunity.
Common viscosity grades and their typical use
| SAE viscosity | Suited to | Notes |
|---|---|---|
| 0W-20 | Modern petrol and hybrid engines | Explicitly required by many Toyota, Honda and Mazda models |
| 5W-30 | The most common grade — recent petrol and diesel | Almost always in a low SAPS version (ACEA C2 or C3) |
| 5W-40 | Diesel and petrol under heavy load or high mileage | Often chosen for turbocharged engines and long runs |
| 10W-40 | Older engines, over 150,000 km | Usually semi-synthetic, with a shorter drain interval |
| 15W-40 | Older diesels, machinery and tractors | Not recommended below -15 °C |
How to choose the right engine oil in 5 steps
The short procedure for identifying the correct oil for your car, using the owner's manual and the canister label.
- 1
Identify the engine
Note the make, model, year and engine code from the registration document or the label in the engine bay. The same model can have two engines with different oil requirements.
- 2
Look up the required viscosity
Open the owner's manual at the lubricants section and note the recommended SAE grade, for example 5W-30. Some manuals give different viscosities depending on climate.
- 3
Note the specification and approval
The same section gives the ACEA or API class and often a carmaker approval such as VW 504.00, MB 229.51 or BMW Longlife-04. The approval is mandatory while the car is under warranty.
- 4
Check the exhaust system
If the car has a particulate filter (DPF on diesel, GPF on petrol) or a sensitive catalytic converter, choose a low SAPS oil from the ACEA C1–C5 classes. High-ash oil clogs the filter.
- 5
Pick the right quantity
Check the sump capacity and add a margin for top-ups. Order the oil together with a new filter — the two are always changed at the same time.
Frequently asked questions
Can I mix two different oils?
Yes, in an emergency. Modern engine oils are miscible, so you can top up with whatever you have to reach a garage. Prefer the same viscosity and specification, and do a full change at the first opportunity.
Does thicker oil protect the engine better?
No. Oil thicker than specified circulates more slowly when cold and can upset hydraulic variable valve timing. The correct viscosity is the one in the manual, not the highest one on the shelf.
Where do I find the specification my car needs?
In the owner's manual, in the lubricants section, and often printed on the oil filler cap. If you don't have the manual, call +373 22 009 350 with the details from your registration document and we'll identify it together.
The information in this guide is indicative. Always follow the intervals and specifications given in your own car's owner's manual.
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