What Engine Oil Does My Car Need?

Two things have to match, and most people only check one.

• The viscosity grade — 5W-30, 5W-40, 0W-20. This is the number everyone looks at.
• The manufacturer approval — VW 504.00, BMW LL-04, Mercedes 229.51, Ford WSS-M2C913-D, Renault RN17. This is the one that actually decides whether the oil is safe in your engine, and it is the one most often ignored.

An oil can be the correct viscosity and still be the wrong oil. “Fully synthetic 5W-30” on a bottle tells you almost nothing on its own — there are many 5W-30s and they are not interchangeable.

Where to find yours: the handbook first, the manufacturer’s online oil finder second, and the service book third. Some cars print it on the oil filler cap. If none of those help, give a garage your registration — that is enough to look it up.

What the numbers mean

Viscosity is resistance to flow, and a multigrade oil is rated at two temperatures.

The number before the W (for winter) is the cold rating — how well it flows on a freezing morning before it has warmed up. Lower is thinner when cold, which means it reaches the top of the engine faster on start-up. Most engine wear happens in the first few seconds after a cold start, so this matters more than it sounds.

The number after is viscosity at 100°C, roughly normal running temperature. Higher is thicker when hot, giving a stronger oil film but slightly more drag.

So 5W-30 and 5W-40 flow identically when cold and differ once hot. That comparison has its own guide, because it is the question we are asked most.

The approvals, and why they are not marketing

There are three layers of specification, and they do different jobs.

API — an American standard, shown as SN, SP and so on for petrol. Broad and fairly basic.

ACEA — the European sequences, and much more useful here. You will see:

• A/B — conventional petrol and diesel engines without particulate filters
• C — catalyst compatible, the low-SAPS oils, for engines with DPFs and modern catalysts. C1, C2, C3 and C5 differ in how thin they are and how far the ash is reduced
• E — heavy duty diesel, not relevant to cars

OEM approvals — the manufacturer’s own, and the one that overrides everything else. These are earned by the oil passing that manufacturer’s specific engine tests, and they appear on the back of the bottle.

Low-SAPS: the part that costs people money

SAPS stands for Sulphated Ash, Phosphorus and Sulphur. These come from the additive package, and they are useful — the anti-wear additive ZDDP contains both phosphorus and sulphur, and it is one of the reasons oil protects camshafts.

The problem is what they do downstream:

• Ash does not burn. Some oil always reaches the combustion chamber, and the ash it leaves behind is trapped by a diesel particulate filter permanently. It cannot be regenerated out. A high-ash oil fills a DPF years early.
• Phosphorus poisons catalysts, coating the precious-metal surfaces and reducing their efficiency.

So low-SAPS oils exist for a specific engineering reason, and using an ordinary high-SAPS oil in a DPF-equipped diesel is not a small economy. It is a slow, invisible way of buying a new particulate filter — and DPF replacement runs to four figures.

It also works the other way: some older engines genuinely want the higher anti-wear content, and putting a very low-SAPS oil in a car with a flat-tappet camshaft is its own problem. There is no universally “better” oil, only a correct one.

The four mistakes we see most

Buying on viscosity alone. The most common by a distance. The grade is necessary and not sufficient.

Assuming a premium brand covers it. An expensive oil without the right approval is still the wrong oil. A cheaper oil that carries the approval is the right one.

Assuming “fully synthetic” means suitable. It describes the base stock, not the specification.

Using a standard oil in a car on a variable service schedule. Long-life intervals are only safe with an oil approved for them, because the oil is being asked to last twice as long. This one is genuinely expensive and it is covered in the long-life servicing guide.

What happens if the oil is wrong

Rarely anything dramatic on day one, which is exactly why it goes unnoticed.

Over months: a blocked particulate filter on a diesel, a catalytic converter losing efficiency, accelerated timing chain wear, and on engines with variable valve timing, running faults and warning lights — the VVT actuators are hydraulic and run on engine oil, so they are unusually sensitive to both viscosity and condition. Deposits build faster, which brings oil sludge forward by years.

And if a warranty claim is ever made on an engine, the oil specification is one of the first things a manufacturer will ask about. Using an oil without the required approval gives them a legitimate reason to decline.

What does it cost?

Nearly the whole spread is the oil itself. A small petrol engine takes about four litres of a widely available grade; a large diesel can need seven or eight litres of a specific low-ash approval costing several times as much per litre. That is not a garage marking up a premium product — it is a genuinely different fluid.

When should you call a garage?

Before you buy oil, if you are topping up and are not certain of the specification. Putting the wrong litre in is easier to avoid than to undo.

If you have already used something you are unsure about, particularly in a DPF diesel — it is worth changing early rather than running it for a year.

Give us the registration and we will confirm exactly what your engine requires, including the approval and not just the grade. We do not substitute a cheaper general-purpose oil to hit a price, because on a modern engine that is not a saving. An oil and filter change to the right specification, seven days a week. If you want the interval question as well as the specification, how often to change engine oil covers it.