If you don't know what engine oil your car needs, it's very easy to make the wrong choice. There are plenty of options available, and it can be overwhelming if you don't know what to look out for. Unfortunately, the wrong decision can cause severe engine damage - so you need to choose the right oil!

That's why we're here to help. By the end of this article, you will know:

  • What the oil grades mean
  • How to find which oil your car needs
  • The differences between semi-synthetic, fully synthetic and mineral oils
  • How long oil lasts
  • How to change engine oil


Subscribe to the BookMyGarage blog today for more helpful tips and advice delivered straight to your inbox


What does engine oil do?

Oil lubricates the moving parts in your vehicle's engine. Combustion generates high levels of heat and friction which oil helps to negate. The metal would smash and grind together as it powered your car if the engine ran dry.

In essence, your car wouldn't run without oil.

So if you don't change your oil regularly, or use the wrong type of oil, you can face a hefty repair bill.


shelf of motor engine oil in auto shop

There's plenty of choice when it comes to engine oil - possibly too much choice if you don't know what the grades mean! Photo by Robert Laursoo on Unsplash


What do the engine oil grades mean?

If you were to look for oil at your local motor shop, you might get overwhelmed. Not only is there a range of choices, but there are loads of numbers on each bottle as well. 5w40, 15w30, 0W30, 10W60 - what does any of that mean?

Well, an oil with numbers either side of the 'W' is known as multigrade oil. This is because they have the ability to cope with seasonal temperature variations. Most of us need multigrade oils and they are the most common type of engine oil. That's because they offer a cost-effective way to keep your car running no matter the temperature. An oil that only has a number in front of the 'W', or no 'W' at all, is known as a monograde oil.

Below is an explanation of what the numbers on a multigrade oil marked '5W40' mean:


5 = Viscosity grade at low temperatures. This is how easily the oil flows through your engine. The lower the number, the easier the oil flows; the higher the number, the thicker the oil. Thicker oil forms a protective film across engine parts.

W = 'winter'. Engine oils marked with a 'W' are more fluid at colder temperatures. This makes your car easier to start during winter. If the oil is not marked with a 'W', it's designed to work better in warmer months.

40 = Viscosity grade at high temperatures. A high grade means that the engine is properly protected in warm weather, but won't be as fuel-efficient. A lower number makes the oil more effective at reducing friction and saving you money at the pump.


So, that's the numbers explained. However, there's still the small matter of working out which type of engine oil your car needs. Your options are fully synthetic, semi-synthetic and mineral - but what are the differences?


What is Mineral Oil?

Mineral oils are refined crude oil. The refining process removes contaminents, unwanted hydrocarbons and other unnecessary natural gunk.

Mineral oil has been around a lot longer than synthetic oils and, being a lot less refined, aren't great for modern cars. They flow through your engine slower than synthetic oils, need to be changed more frequently and increase your fuel consumption. None of which is ideal.


What is Fully Synthetic Oil?

Fully synthetic oil is a highly refined, chemically modified performance oil. It has less impurities than mineral oils and includes plenty of additives to boost its lubricating power.

Fully synthetic oil is way more efficient than mineral oil. It also offers plenty of advantages, which include:

  • Less frequent need to change
  • Less wear on engine parts
  • Increase engine lifespan

Unfortunately, this does come at a cost. Fully synthetic oil is more expensive than mineral oils.


What is Semi-Synthetic Oil?

Semi-synthetic oil sits between the two. The formula isn't as advanced as fully synthetic oil, but is still more refined than mineral oil. This means that semi-synthetic oil is a cost-effective option for many drivers.


What type of engine oil does my car need?

Most modern cars use semi-synthetic oil. They can run on fully synthetic and mineral oils, but there are plenty of disadvantages to doing so.

Drivers of older vehicles will need to use mineral oils. These can still provide high-quality lubrication despite their issues, especially in classic cars. Many manufacturers and mechanics recommend only using mineral oils in these vehicles.

That's because each engine is designed to run a specific oil grade and viscosity. Unless your vehicle is heavily performance modified, you should NOT stray from the recommended grade and viscosity of oil. This may mean that you have to pay a little extra, or change oil more often, but also get the full performance from your car.



How will I know what oil to put in my car?

You can find which engine oil your car needs in your vehicle handbook. This will give you the recommended viscosity, grade and, in some cases, the best type to use.

Some manufacturers develop their own oil specifications. These include VW, Mercedes, BMW and GM.

If your vehicle handbook recommends manufacturer's specification oil, you MUST buy an oil that satisfies this. Specialist oil can cost a little extra, but they are 'long life' oils, designed to support long intervals between services. This saves you money in the long run - which is always handy!

Your handbook might give an acceptable alternative for use in emergencies, but you should stick to the recommended oil as much as you can.

If you can't find the information in your vehicle's handbook, you can enter your postcode into an online tool, such as Opie Oil's. 


How to change engine oil



If you check your oil (see above) and realise it needs changing, here's how you can do it yourself.

  1. Remove the sump plug and drain oil. You can find this on the underside of your car (so you will need to raise it off the ground).
  2. Replace the sump plug once all the oil has drained.
  3. Refill the oil to the correct level. Following the manufacturer recommendations for grade, type and quantity of course.


However, an oil change on its own isn't very helpful. Oil flows through an oil filter, which catches any debris, dust or bits of metal oil has picked up while moving through the engine. As a result, this filter gets dirty very quickly.

If you don't change your oil and oil filter together, your oil will become less effective much sooner! This can lead to problems in the future, including increased fuel consumption and more frequent oil changes.

If you can't change your oil and oil filter together, book an oil and filter change at a garage near you and get a professional to do it for you.


How much oil does my car take?

The amount of engine oil your car needs can range from 3 litres all the way up to 12 litres.

Generally, the larger the engine, the more oil your car needs.


Engine Size







Average quantity of oil used in an Oil and Filter Change

3.5 litres

4.9 litres

5.2 litres

6.1 litres

6.4 litres

8.1 litres


If you choose a professional mechanic to complete your oil change, they will have access to the exact amount of oil your car needs. This saves you any extra stress that over or underfilling your engine oil can cause.


How long does engine oil last?

Most manufacturers recommend an engine oil change every 5,000 miles for modern cars. If your car runs on older mineral oils, this may be as often as every 3,000 miles but for fully synthetic oils, you could drive 10,000 miles between oil changes.

The BookMyGarage service schedule always includes an oil change as standard. This includes a full drain and refill of your engine oil, as well as a fresh oil filter. When you book a service with one of our UK garages, you can rest assured that your car gets the right oil, as well as the full amount it needs.


fresh golden engine oil being poured from green bottle into car oil tank

Engine oil must be golden in colour to provide maximum effectiveness. If you notice that yours is dark brown in colour or full of lumps, you need to book an oil change as soon as possible.



There's nothing more important than choosing the right engine oil for your car. Your engine is designed to use a specific oil grade and viscosity and a certain type of oil. Most modern cars use a multigrade, semi-synthetic oil, whereas older cars are designed to use mineral oils. You can identify a multigrade oil by checking if it has numbers either side of the 'W'.

Engine oil should last 5,000 miles between oil changes, and you must change the oil and oil filter together. Else, your oil will be less effective and need changing more frequently. You can book a professional oil change with one of thousands of UK garages here.