5W-30 vs 5W-40: Which Oil Is Right?

They flow identically when cold. They differ once hot.

Both are 5W, so on a freezing morning they behave the same. The second number is viscosity at 100°C — roughly normal running temperature — so 5W-40 keeps a thicker oil film once the engine is up to temperature, and 5W-30 is thinner and slightly freer.

Which one your engine takes is not a preference. It is set by bearing clearances, oil pump output and, on modern engines, by how fast the variable valve timing has to move. Use what the handbook specifies — and check the manufacturer approval as well as the grade, because the approval matters more than the numbers.

What the numbers actually measure

The grades come from SAE J300, and a multigrade oil is rated at two temperatures.

The number before the W — W for winter — is a cold-flow rating, measured at low temperature. Lower is thinner when cold. This matters more than most people realise: the large majority of engine wear happens in the first few seconds after a cold start, before oil has reached the top of the engine. A 0W reaches the camshaft faster than a 5W, which reaches it faster than a 10W.

The number after the W is viscosity at 100°C. Higher is thicker when hot.

A multigrade manages both because it contains viscosity index improvers — long-chain polymers that stay coiled when cold and uncoil as they warm, thickening the oil at temperature. That is also why oil shears over time: those chains break down, and an old 5W-40 gradually stops being a 40.

What changes when you go thicker

	5W-30	5W-40
Cold start protection	Identical — both are 5W
Oil film at running temperature	Thinner	Thicker
Internal drag	Lower	Higher
Fuel economy	Marginally better	Marginally worse
Oil pressure at hot idle	Lower	Higher
Flow through narrow galleries	Faster	Slower

Thicker is not better. It is a trade, and the manufacturer has already made it.

Why manufacturers choose one over the other

Bearing clearances. An engine built to tight tolerances needs an oil thin enough to get into the gap. Too thick and it does not fill the clearance properly at the rate the crank is turning.

Oil pump design. The pump is sized for a specific viscosity. Feed it something thicker and pressure rises, flow falls, and the pressure relief valve spends more of its life open.

Variable valve timing. This is the one that catches people out on modern engines. VVT actuators are hydraulic and move using engine oil, and they are designed around a specific viscosity. Oil that is too thick slows their response, and the ECU notices the discrepancy between commanded and actual cam position. The symptom is an amber warning light on the dash and a cam timing fault code on an engine that is mechanically fine.

Emissions and economy targets. Thinner oils reduce pumping losses, which is why the industry has moved towards 0W-20 and thinner still.

When 5W-40 is the right answer

• The handbook says so — which is the only reason that really counts
• Many European diesels, particularly older turbodiesels running higher cylinder pressures
• Some performance and high-output engines
• Sustained very hot operating conditions or heavy towing, where a manufacturer offers it as an alternative grade

Note the pattern: these are engines designed around a 40. It is not a general-purpose upgrade.

“Thicker oil will stop my engine burning oil” — is that true?

Partly, and it is worth being straight about the trade-offs rather than dismissing it.

On a worn engine with increased bore or valve guide clearance, a thicker oil does slightly reduce consumption, because it is marginally harder to squeeze past the rings and down the guides. As a temporary measure on an old engine that is otherwise sound, it is not unreasonable.

What it does not do is fix anything. And there are three real costs:

• It usually breaks the manufacturer approval, which matters for warranty and, on a DPF diesel, for the ash content of the oil
• It can slow VVT response and set cam timing faults
• It masks a symptom you should be measuring, because the rate of consumption is diagnostic in itself

If the car is using oil, establish how much and find out where it is going first — why a car burns oil covers the four routes and how to tell them apart, and when the blue smoke appears narrows it further.

Can I mix them, or switch?

Mixing in an emergency: yes. If the level is low and the correct grade is not available, topping up with a different multigrade is far better than running the engine low on oil. Get it corrected at the next change.

Switching permanently: only within what the manufacturer allows. Some handbooks list more than one acceptable grade, sometimes with a temperature range. If yours lists only one, that is the answer.

Going thinner than specified — putting 5W-20 in an engine that wants 5W-40 — is the genuinely risky direction, because it reduces film strength where the engine was designed to have it.

Why are new cars on 0W-20 and thinner?

Tighter manufacturing tolerances, better bearing materials, and hard fuel-economy targets. Every bit of drag inside an engine costs fuel, and thinner oil is one of the easier places to remove it.

The important consequence: on a car specified for 0W-20 you cannot “improve” matters by using something thicker. The clearances are not there for it, the VVT is not designed for it, and the pump is not sized for it.

What does it cost?

Oil type	Typical cost fitted
Semi-synthetic, small petrol engine	£49–£75
Fully synthetic, common grades	£75–£109
Low-SAPS / manufacturer-approved specification	£99–£129

Grade barely moves the price. What moves it is capacity and approval — a large diesel taking eight litres of a specific low-ash oil costs several times what a small petrol engine costs on four litres of a common grade.

When should you call a garage?

Before switching grades, if you are thinking about it because the car is using oil or because a forum suggested it. There is usually a better answer.

If a cam timing or VVT fault has appeared after an oil change, mention what went in. It is a genuinely common cause and an easy one to correct.

Give us your registration and we will tell you the grade and the approval your engine needs. Oil and filter changes in Birmingham, seven days a week. If you want the interval as well, how often to change engine oil covers it.