If you’re not clued-up, you’re probably wondering what on earth synthetic engine oils are. What makes them synthetic, and what are the pros and cons when compared to regular engine oil?
Whilst regular engine oil is made from circa 70-90% of base oils and additives, synthetic oil tends to use more distilled crude oil as a base (around 20-30%) before petroleum components and additives are mixed in. Brands keep it a trade secret of the other exact ingredients, but they are still quite popular with motorists in high-mileage cars.
The main thing to note is that synthetic oil is more expensive than regular oil, but it tends to have longer oil change intervals. It can be had in the same grades as regular oil, such as 5W-30, but synthetics flow better at cold temperatures and are more resistant to oxidisation and chemical degradation. However, you do have to compromise with marginally lower fuel economy at motorway speeds. And it tends to separate additives that you may add.
The best synthetic engine oils 2024 at a glance:
The best synthetic engine oil: Carlube Triple R R-Tec 5W-30 – Buy from Amazon.
Editor’s pick: Castrol Edge 5W-30M – Buy from Amazon.
Best synthetic 5W-40 engine oil: Liqui Moly Synthoil High Tech 5W-40 – Buy from Amazon.
We’re here to guide you on what the best synthetic engine oils are on the market. We’ll also inform you on the oil change intervals and what application they’re best suited to. In many ways, they can be superior to the regular stuff you might’ve been using for years, such as a bottle of 10W-40 regular engine oil.
How to find the right synthetic engine oil for your vehicle
Find the correct engine oil for your vehicle
Pros
- Easy to search
- Lots of options to choose from
Cons
- Delivery isn't free if you're not a Prime member
The best synthetic engine oils 2024
The best synthetic engine oil
The range of 5W-30 Carlube R-Tec oils are designed with a number of engines in mind, from VW Group cars up to Mercedes and Jaguar Land Rover units.
Pros
- Strong alternative for 5W-30 users
- Range of different versions available
- Helps keep the engine clean, protects components and run smoothly
Cons
- Range of cars in mind is fairly limited
Editor's pick
It's a great option for protecting regularly-used, higher-mileage engines, as it can take a lot of pressure. It's even recommended by some manufacturers such as BMW.
Pros
- Approved by some manufacturers
- Perfect for a wide range of engines
- Ideal for protecting a higher-mileage engine
Cons
- It won't be suitable for every engine
- Isn't the best for the highest of temperatures
The best synthetic 5W-40 oil
It's recommended for quite a lot of Volkswagen engines particularly, which of course, will stretch to Skodas, Seats, and Audis as well. Liqui Moly only disclose those as an example, but you can always check on Amazon Garage to see if your vehicle can take it. It's suitable for both petrol and diesel engines and helps them run just that little bit more smoothly as well as protecting the internal components.
Pros
- Offers strong, ample protection against friction and wear
- Suitable for both petrol and diesel engines
- Better value per litre than the Castrol Edge oil
Cons
- Won't be suitable for every engine
Best for diesel engines
It's on some middle ground in terms of pricing, but you should be aware that it's primarily designed for trucks and larger diesel-engined vehicles.
Pros
- Designed to be very resistant to contaminants
- Will last a while and help protect a hard-working engine
- Pretty good value at just over £10 per litre
Cons
- Primarily for diesel-engined trucks
Best value 5W-40 synthetic oil
It's also said to improve an engine's overall performance over standard oils. It's the ideal stuff to use once an engine racks up the miles, as it's recommended for use in vehicles with over 75,000 miles. It's also compatible in both diesel and petrol engines, which is a neat bonus as well.
Pros
- Great protection against high and low temperatures
- Ideal for cars with over 75,000 miles
- Protects the engine against high stress and oil sludge
Cons
- Only available in that 5W-40 grade
- Not really designed for low-mileage cars
Best value synthetic oil
It comes in a 2-litre bottle - which is a tiny amount if you're due an oil change. But the price cut compared to some other synthetic oils means you can either buy a load of them in bulk, or simply use it to top up your existing oil level on your car. The benefit of the latter being you have a little bit of strength flowing throughout your engine.
Pros
- Great value for money at £9 per litre
- Useful for a top-up on your engine's existing oil
- Great for higher mileage vehicles
Cons
- 2-litre bottle won't fill a whole engine
- Part synthetic oil will never be superior over full synthetic
Things to remember about synthetic engine oil
If topping up, can I mix synthetic oil with existing conventional oil?
Ideally, synthetic oil is at its best when it’s the only thing inside your engine. However, the answer is very much yes. If anything, mixing in the synthetic effectively adds additives to the regular oil, which may help your engine run more smoothly and cleanly. But it won’t be as effective as a full oil change.
Why is synthetic engine oil more expensive?
Simple: a whole load of additives and, you could say, secret ingredients are used to make them. Conventional oils use less of this stuff and is therefore easier to purify and produce. They really came into their own throughout a number of events in history when oil was rationed. A great example being during the two Oil Crises in the 1970s, which effectively forced manufacturers to look into synthetic products, minimising the use of base crude oils. When changing the stuff, you should also changing the oil filter, to ensure the engine runs cleanly.
You’ve mentioned high mileage cars a few times, are synthetics better for mile munchers?
Some synthetic oils are massively effective for them. This is because along with the additives, they tend to come with extra seal conditioner which protects worn seals. This helps with minimising oil leaks as well which can only be a good thing. But whilst engine oils are one thing, other vital oils can be quietly forgotten. Read up on our list of the best gearbox oils.
How to top up engine oil
Aaron Hussain is a commercial content writer at Bauer Media writing for Parkers and CAR. He is obsessed with classic cars and anything with a fascinating story to tell.
Sign up to the Parkers Newsletter to keep up to date with more of the latest reviews, news, and recommendations from the Parkers team.
Just so you know, while we may receive a commission or other compensation from the links on this page, we never allow this to influence product selections – read why you should trust us
