Engine Oil

Which engine oil grade does your car need?

Your owner's manual is the starting point, not the shelf. The viscosity grade printed there is the one your engine was designed around, and substituting a different grade, even a neighbouring one, is only safe if your manual explicitly lists both as acceptable. The "W" in a grade stands for "winter" and describes how the oil flows at cold start; the second number describes viscosity at operating temperature.

A few clarifications that come up often:

• 5W30 vs 5W40: 5W40 is not automatically better. It provides a thicker film at operating temperature, which suits certain older or higher-load engines. For a modern engine specified for 5W30, using 5W40 can be too thick and is only appropriate if your manual permits it.
• 0W30 vs 5W30: These are not interchangeable by default. Use 0W30 only if your manual lists it as acceptable; otherwise stay with the grade specified.
• 0W20 vs 5W30: Neither is universally superior. A modern engine designed for 0W20 should not be filled with 5W30. The correct grade is the one your manual specifies, not the one that happens to be on the shelf.
• 10W40 vs 5W40: 10W40 is for older engines (pre-Euro 5) without a DPF, where the manual permits it and you are experiencing some oil consumption. 5W40 suits more modern turbodiesel and higher-load engines with the appropriate OEM approval.

If your manual lists a range (for example, both 0W30 and 5W30), any grade within that range is acceptable. Staying outside it is not.

Does your engine need a low-SAPS oil for its DPF or catalytic converter?

SAPS stands for sulphated ash, phosphorus, and sulphur. These compounds occur naturally in engine oil additives, but in a diesel particulate filter or a modern catalytic converter, they accumulate and accelerate clogging. The ACEA classification system divides engine oils into A/B class (full-SAPS, for older engines without sensitive emission systems) and C class (low- or mid-SAPS, for modern engines with DPF or specific catalyst requirements). For a full explanation of what each ACEA and API code means in practice, a dedicated knowledge article goes into more detail.

Use a low-SAPS (ACEA C2 or C3) oil if:

• Your diesel engine has a particulate filter (DPF).
• Your car is Euro 5 or Euro 6 (petrol or diesel) and the manual asks for a C-class oil.
• Your manual mentions "low-ash" or lists specific OEM approvals such as VW 504.00/507.00 or MB 229.51.
• Your car runs on modern diesel with a DPF, regardless of whether you also use E10 petrol in another car. For E10 petrol engines, oil selection still follows OEM approvals and ACEA spec; E10 fuel itself does not change the oil grade, but many modern petrol engines that run on E10 also fall under Euro 5 or 6 and therefore require C-class oil.

Full-SAPS oil (ACEA A3/B3 or A3/B4) is acceptable if:

• Your engine is older, pre-Euro 4 or 5.
• It has no DPF and a traditional catalytic converter that does not require low-ash protection.
• Your manual specifies an A or B class oil and makes no mention of C-class or low-ash requirements.

Using a high-SAPS oil in a DPF-equipped engine accelerates filter clogging and can lead to expensive regeneration failures or a full filter replacement. This is not a theoretical risk: it is a documented cause of premature DPF failure. If you are unsure whether your car has a DPF, check your manual or ask your garage before choosing an oil.

Full synthetic, semi-synthetic, or mineral: which does your engine actually require?

Check your manual first. Many modern engines specify full synthetic as a requirement, not a preference. For older engines, the picture is different.

Full synthetic is not automatically the better choice for every engine. For older engines and classic cars whose manuals specify mineral oil and short intervals, mineral oil is still the correct option. Switching to full synthetic in those cases does not improve protection and may be unnecessary.

For high-mileage engines, switching from mineral or semi-synthetic to full synthetic is only safe if the new oil carries exactly the same viscosity and OEM specification as the oil already in the engine. A different spec can soften old deposits and dislodge them into oil passages, or disturb seals that were conditioned to a different base oil. If your engine has very high mileage and you are considering a switch, it is worth asking a garage before you do it.

How often should you change engine oil?

• Follow the oil change interval specified by your manufacturer. For most modern passenger cars running on long-life synthetic oil, check your manual for the correct interval.
• Change your oil at least once per year, even if you have not reached the kilometre threshold. Short trips and cold starts cause fuel dilution and condensation in the oil that degrade it faster than mileage alone suggests. This applies especially if you drive relatively few kilometres per year.
• Check your oil level on the dipstick regularly, or at every fuel fill for older engines. Do not wait for a warning light to discover a low level.
• Always replace the oil filter when you change the engine oil.

Driving too long on old oil is not immediately obvious. The oil thickens or thins, loses its additives, and can form sludge over time. You may not notice any difference in how the car drives until the damage is already done.

If you change your oil yourself, follow a dedicated step-by-step guide rather than relying on general instructions. If you are unsure about any part of the process, a garage can do it quickly and correctly. Before changing oil in a very dirty engine, a flush additive can help dissolve internal deposits; the Engine Flush product page explains when and how to use it.

Can you top up with a different brand or grade?

For a one-off top-up between services, you do not always need the exact same bottle as last time, but the specification must match.

Mixing rules:

• For an emergency top-up, you can use a different brand, provided the viscosity grade and ACEA or OEM specification match what is already in your engine.
• Do not mix different grades or specifications as a routine. If you had to mix once out of necessity, plan an early full oil change.

Normal oil consumption:

Some oil consumption between services is within the acceptable range for many petrol and diesel engines. If your engine consistently uses a notable amount of oil, that warrants a garage inspection. Oil alone will not resolve the underlying cause, whether that is worn piston rings, valve stem seals, or a leak.

To check your oil level correctly: park on level ground, switch the engine off, wait a few minutes, then pull the dipstick. The level should sit between the minimum and maximum marks. Your owner's manual specifies the fill quantity for your engine; do not use a generic figure.

Avoid overfilling. If the level is above the maximum mark on the dipstick, do not continue driving. Excess oil can foam under pressure, reducing lubrication, and oil mist drawn into the intake can damage the catalytic converter or DPF.

Unopened oil can generally be stored in a cool, dry place without losing quality; for more detail on storage and shelf life, a knowledge article covers this separately.

When topping up is not enough: go to a garage

Stop driving immediately and contact a garage if you notice any of the following:

• The oil pressure warning light comes on while driving.
• You hear persistent metallic knocking or tapping from the engine.
• You see blue smoke from the exhaust (a sign of burning oil).
• The oil on the dipstick looks milky or creamy (possible coolant contamination) or smells strongly of fuel.
• Your oil level drops rapidly despite topping up regularly.

Topping up will not solve any of these problems. They point to mechanical issues that require professional diagnosis, not a different oil grade or a larger top-up.

On overfilling: if the level is above the maximum mark on the dipstick, do not drive further. Have a garage drain the excess. The risk of foaming, reduced lubrication, and damage to your catalyst or DPF is real and not worth the short journey to the workshop.

If you are not sure whether your situation calls for a top-up or a garage visit, the answer is straightforward: when in doubt, stop driving and ask a professional. Engine oil is a maintenance fluid, not a repair tool.

If you are still deciding between two grades and both appear in your owner's manual as acceptable, choose the lower viscosity of the two for everyday mixed driving, or check the individual product pages for the OEM approvals that match your car's make and model.