What’s the difference between 5w30 and 5w40 engine oil?

What’s the difference between 5W30 and 5W40 engine oil? Which one should I choose for my vehicle? These are questions that often come up when it comes to changing or upgrading your vehicle’s engine oil. We’ll help you understand the differences between these motor oils to make the best choice.

5W30, 5W40, 10W40… What does this mean?

5W30, 5W40, 10W40… These are numbers and letters that may seem abstract at first. Displayed on motor oil cans, these “references” represent what is known as the viscosity index. In short, this index will tell us about the temperatures the oil is capable of withstanding:

• associated with the letter W (for “winter”), the number on the left indicates the fluidity when cold: to put it simply, the lower this number, the more liquid the engine oil is and the faster it flows at low temperatures.
• the number after the W corresponds to the hot viscosity: it indicates the fluidity of the oil at a normal engine operating temperature. The higher this viscosity grade, the denser the oil and the slower it flows.

As you can see, oil flow depends on engine temperature. In practical terms:

• A viscosity grade of 5W will ensure good cold lubrication when starting the vehicle, even in extreme weather conditions (down to -30°C!). Conversely, a 10W oil will be less effective when starting in cold weather.
• A high hot viscosity grade (e.g. 30 or 40) will protect the parts of the engine that are subject to the most stress when running.

Following these explanations, you may be thinking that 5W30 and 5W40 oils are technically quite similar. In reality, each of these oils has its own specific characteristics.

What is the difference between 5W30 and 5W40 oil?

Before we talk about the differences between the 5W30 and 5W40 oils, let’s look at what they have in common:

• these are two synthetic oils: they are chemically modified and their composition includes additives (antioxidants, anti-wear, anti-corrosion, detergents…) that make them more effective than so-called “mineral” (conventional) oils
• these oils are intended for the engines of new vehicles (diesel or petrol)
• they can be used all year round, in both winter and summer
• they require oil changes less frequently than traditional oils: every 5,000 to 7,500 miles
• 5W30 and 5W40 oils also help to limit the formation of deposits
• they protect the engine’s sensitive parts, helping to extend its lifespan
• they are suitable for all types of journeys (city, road and motorway).

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Finally, what distinguishes a 5W30 oil from a 5W40 oil? Certain properties related to their hot viscosity:

• 5W30 engine oil:
  • is more liquid when hot, allowing it to circulate more easily between engine parts
  • is suitable for vehicles fitted with particulate filters (DPF) as it prevents their clogging
  • helps to save fuel
  • helps to limit pollutant gas emissions
• 5W40 motor oil:
  • is thicker when hot, and therefore makes it easier to seal any cracks
  • is suitable for vehicles without DPF
  • is suitable for sporty driving and the engines of powerful cars
  • provides increased protection for the catalytic converter
  • is generally less expensive to purchase.

Here are the two motor oils we recommend (one 5W30 and one 5W40) for their superior quality and attractive price:

- 37%

Amazon.co.uk

★★★★★

Castrol EDGE 5W-30 LL Engine Oil 5L

£40.17 £64.36 

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- 32%

Amazon.co.uk

★★★★★

Castrol EDGE 5W-40 Engine Oil 5L

£41.39 £61.62 

View deal

Last updated on 10/10/2024 21:39

Now that you know the differences between these two oils, let’s find out how to choose between 5W30 and 5W40.

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5W30 vs 5W40: which is best suited to your engine?

Choosing the right motor oil for your vehicle can’t be done at random, to avoid premature wear or engine malfunction. In fact, while our advice may give you an idea of the oil that seems to be suitable for your car, we recommend that you scrupulously follow your car manufacturer’s recommendations on the subject.

How do you find the right oil for your engine?

There are several ways of finding the most appropriate type of oil for your vehicle:

• Check your vehicle owner’s manual: in the “Maintenance” section, you’ll generally find the type of oil recommended by the manufacturer.
• Find your number plate on a site such as Amazon: the search engine will select the list of motor oils compatible with your vehicle for you.
• Contact your car dealer or garage: they will be able to advise you on the most suitable oil.

What engine oil for my car?

What criteria should you take into account when choosing your oil?

To make the best choice for your engine oil, there are three main criteria to consider:

1. The temperature of your region in winter: it’s common for manufacturers to recommend two types of oil, with one being better suited to extreme winter weather conditions. For example, if your manufacturer recommends a 0W30 and a 5W30, you should be aware that the 0W30 will make it easier to start up in very harsh winters (particularly if you live in the mountains). On the other hand, the 5W30 will be suitable in most cases.
2. The characteristics of the oil recommended by your manufacturer: in the owner’s manual, you’ll find the viscosity index and the manufacturer’s standards you need to comply with when choosing your engine oil. The standards give details of the quality of oil to choose (e.g. API SN/CF, ACEA C3, VW 505.00, etc.).
3. The amount of engine oil needed : whether you want to change or top up your oil, we recommend you opt for a 4 or 5 litre can. If you need to top up, it’s not uncommon to have to add more oil in the months following the first addition.

A few important notes to refine your choice of motor oil:

• Your manufacturer recommends a specific brand or reference of motor oil? While it’s best to stick to the recommended brand, you should know that it’s not compulsory to follow this advice. In fact, your choice should above all be based on the characteristics (or standards) of the oil mentioned in the maintenance guide. You’ll find the list of standards with which an oil is compatible on the label affixed to each can.
• Oils with the same viscosity index can be mixed together without causing any problems. Just make sure the oil you choose meets your manufacturer’s requirements.

Can a 5W30 oil be mixed with a 5W40?

Do you need to top up with 5W30 engine oil, while you still have 5W40 at the back of the garage (or vice versa)? While, in theory, it is possible to mix two different motor oils, you should know that this is strongly discouraged by manufacturers. This is because mixing two lubricants of different quality and/or viscosity can have harmful consequences:

• the oil obtained will be of degraded quality, as it will be given the properties of the lowest quality oil
• Using the wrong oil on a modern engine can cause important damage to the engine and exhaust system (DPF, catalytic converter, etc.).

So you can understand why it’s best to avoid using two different engine oils. Always use the type of oil recommended by your manufacturer, even if you’re just topping up the tank.

If you have mixed a 5W30 oil with a 5W40, we advise you to drain the oil tank completely as soon as possible. It will then be necessary to refill the tank with an oil suitable for your vehicle.