The difference between 5W30 and 5W40 oil

The choice between 5W30 and 5W40 oil keeps many drivers awake as they stand in front of the shelf at the parts shop. The two markings look almost identical, differing only in the second number, yet they trigger hours of arguments on online forums. The truth is that both oils behave exactly the same during a cold start, because both classes begin with “5W”, but they differ in viscosity once the engine warms up. In this article we explain what that difference really means, when to pick one class over the other, whether you may swap them, and how the decision about oil thickness translates into fuel consumption and real engine protection.

What is the difference between 5W30 and 5W40?

The whole secret lies in the second number of the marking. The first part, the “5W” shared by both oils, describes cold behaviour — the letter W comes from the English word Winter. Since both classes have a five here, both 5W30 and 5W40 handle a cold morning start equally well and build oil pressure just as quickly after the engine fires up. The difference only begins once the oil warms up to operating temperature, which is around 100 degrees Celsius.

It is the second number, “30” versus “40”, that tells you how thick the oil stays when hot. The 5W40 class keeps a higher viscosity at high temperature than 5W30, meaning it forms a slightly thicker lubricating film between the moving parts. The gap is not huge, but in practice it can decide whether the oil is a perfect match for your engine’s design or only a rough approximation of what it needs.

The second number, or hot viscosity

Viscosity is, put simply, the resistance with which an oil flows. The higher the second number, the thicker the oil remains at full operating temperature and the thicker the film it maintains in the most heavily loaded spots, such as the crankshaft bearings. A thicker oil film can seal better and separate metal surfaces more fully in a hot, heavily loaded engine. This comes at a price, though: a thicker oil creates more internal resistance, so the oil pump and the whole system have to work a touch harder.

• 5W30 — thinner when hot, lower internal resistance, favours reduced fuel consumption
• 5W40 — thicker when hot, a heavier lubricating film under high load
• When cold both classes behave identically, because both start with “5W”
• Higher viscosity does not automatically mean better protection — the fit is what matters
• The right choice is set by the engine design, not by the feeling that thicker is always safer

When to choose 5W30, and when 5W40?

There is no universal answer that would suit every car, because the right viscosity depends on the specific engine. We can, however, point to typical scenarios in which one class works out more often than the other. Treat the guidance below as a general orientation, not as a replacement for your vehicle’s owner’s manual.

• 5W30 — modern, economical engines in which the maker deliberately chose low friction
• 5W30 — units requiring specific fuel-saving approvals and working with a particle filter
• 5W40 — older engines with larger operating clearances and higher mileage
• 5W40 — heavily loaded units, high-power cars, frequent driving in heat or with a trailer
• 5W40 — engines that begin to consume oil, where a thicker film can help

Can 5W30 and 5W40 be used interchangeably?

This is the most common driver question and also the most misleading. Swapping is only safe when your engine’s manufacturer explicitly permits both viscosity classes. Many owner’s manuals list several options depending on climate and driving conditions — if both values appear in your handbook, you have a free hand. But if the designer specified strictly one class, changing to a thicker or thinner oil on your own can do harm over time.

Topping up with a small amount of an oil from the neighbouring class, when you run out of the correct product on a trip, usually will not cause a failure. Constantly running an oil other than the recommended one is a different story. In an engine designed for 5W30, permanently pouring in thicker 5W40 will raise resistance and consumption, while in a unit that needs 5W40 a thinner oil may fail to hold a sufficient film when hot. The starting point should always be the manual, not a substitute grabbed at random off the shelf.

Manufacturer approval matters more than viscosity alone

The key truth that every good mechanic repeats is this: the viscosity class alone is not everything. Two 5W30 oils can differ dramatically in base quality, additive package and, above all, in the engine maker’s approval, such as VW, BMW or Mercedes. It is precisely the manufacturers’ approvals that in practice tend to matter more than thickness, and they decide whether a given oil truly suits your car, including the protection of systems such as the particulate filter.

1. Find the recommended viscosity class in the owner’s manual or service book
2. Check whether the maker permits several options depending on climate and load
3. Beyond viscosity, read the required standard and approval — that is equally important
4. When buying oil, make sure the can meets both the right viscosity and the approval
5. If in doubt, ask a mechanic or use a professional oil selection

A thicker oil does not always mean a better one — what counts is compliance with your engine’s specification, not the number on the can.

The effect on fuel consumption and engine protection

The difference between 5W30 and 5W40 has a purely practical dimension. The thinner-when-hot 5W30 creates less internal resistance, so the engine loses less energy overcoming fluid friction, and that translates into slightly lower consumption. This is exactly why modern, economical units are often designed around a lower viscosity. The fuel-consumption differences are not dramatic, but over a year and thousands of kilometres they can be noticeable.

On the other hand, the thicker film of 5W40 can better protect a hot, heavily loaded engine and seal units with larger operating clearances more effectively. That is why older cars with higher mileage often feel better on a thicker oil. Protection and economy are always a compromise — and where its optimal point lies for your car was already established by the engine designer and written into the recommendations.

Common myths about choosing 5W30 and 5W40

A fair amount of misunderstanding has grown up around the difference between these two classes, and it can cost a driver unnecessary money or even the health of the engine. The first and most persistent myth holds that the thicker 5W40 will always protect the unit better, just to be on the safe side. In reality, an oil that is too thick for the design creates more resistance, reaches the furthest corners more slowly once hot, and raises fuel consumption. The maker matched the viscosity to specific clearances and oil-pump capacity, so departing from the recommendation usually harms rather than helps.

The second misunderstanding assumes that since both classes share the same “5W”, they are essentially interchangeable and it makes no difference which one you pour in. True, they behave identically when cold, but it is the hot viscosity that decides the fit to a warm, loaded engine, and that is exactly where they differ. The third myth insists that an older car should “always” get a thicker oil. That can be true with large clearances and high mileage, but it is not a rule — if the maker still recommends 5W30, switching to 5W40 on your own is no guarantee of improvement.

The fourth misconception concerns season and marketing. Some drivers believe that a higher second number is a marker of a “stronger” or pricier, and therefore better, oil. Yet the number speaks only about hot thickness, not about base quality, additive package or compliance with an approval. A well-chosen 5W30 can protect an engine better than a random 5W40 without the right specification. The soundness of the choice is always decided by the match to the car’s requirements, not by the number on the can.

Frequently asked questions about the difference between 5W30 and 5W40

Is 5W40 better than 5W30?

There is no objectively better oil — there is only an oil better matched to a specific engine. 5W40 forms a thicker film when hot, which can be an advantage in older and heavily loaded units, but in a modern engine designed for 5W30 that same thickness will raise resistance and consumption, and sometimes break the approval requirements. The best oil is the one the maker of your car explicitly expects.

Can I pour in 5W40 instead of 5W30?

Only if your car’s owner’s manual permits both classes. Both oils start identically when cold, so the start will not suffer, but 5W40 is thicker when hot. In an engine intended strictly for 5W30 this can raise internal resistance and consumption, and sometimes clash with the required fuel-saving approval. Before you reach for the thicker oil, check exactly what appears in the service book.

Does the viscosity difference depend on the oil base?

The number alone speaks only about viscosity, not about whether the oil is fully synthetic or semi-synthetic. In both the 5W30 and 5W40 classes you will find products of varying base quality, although modern oils with manufacturer approvals are almost always synthetic products. The base type and additive package influence durability and engine protection at least as much as thickness does.

Will 5W40 help when the engine consumes oil?

In older, higher-mileage units a thicker 5W40 can help, because the heavier film seals worn parts better and can limit oil loss. This is a symptomatic fix, however, not a repair of the cause. If the engine clearly burns oil, it is worth finding the source of the problem instead of masking it with a viscosity change alone, especially when the maker did not foresee that oil.

Is 5W enough for winter in a cold climate?

Yes, the 5W winter class gives a very good cold start in typical hard frosts and is entirely sufficient for most drivers. Since both 5W30 and 5W40 begin with a five, their cold behaviour is practically identical. The choice between these classes is therefore settled not in winter at start-up, but in summer and during demanding driving, when hot viscosity is what counts.

Not sure which viscosity class fits your engine? During a mobile oil change we will advise on the right choice between 5W30 and 5W40, select a product with the correct approval, and carry out the change with call-out to your address, with no workshop visit needed.