Choosing the best oil for your car's engine can be tough, given the large number of options available. Our guide can help you make the right choice
29 August 2016

Oil is one of those mysterious substances for many car owners.

Unless you’re an enthusiast, you may well open your car’s bonnet only when something goes wrong – you’ve run out of washer fluid or there’s an alarming amount of smoke coming from somewhere. But keeping the car well lubricated is one of the most important maintenance jobs, because unwanted friction in a fast-moving engine usually leads to bad, potentially expensive news.

Advanced mechanical technology in modern cars means that modern oils are more complex than ever, according to David Wright, director general of the United Kingdom Lubricants Association.

He says increased regulations on vehicle emissions and increasing demands from consumers for performance uncompromised by advanced fuel economy have meant that the lubricant industry of oil blenders and marketers has had to keep up with manufacturer demands as they chase better efficiency.

“Smaller oil sumps mean we are all using less oil each year but the oil we are using has to work twice as hard,” Wright says. “Car manufacturers today are demanding thinner and lighter engine oil viscosities to achieve enhanced fuel consumption in a smaller, more powerful engine while at the same time reducing emissions.”

This means that getting the right oil for your car is more important than ever. Get it wrong and while you won’t see instant disaster as you would if you’d put diesel in a petrol car, you will subject your engine to excess component wear.

“You won’t see an issue immediately. It’s not like putting contaminated fuel into your vehicle,” says Wright. “But within 20,000 miles, you could have severe operational issues.”

What kind of oil you need depends on the engine. All new cars will come with a recommended grade listed in the manual and usually a recommended manufacturer, too. The grade will be shown in numbers, separated by the letter ‘w’ – for example, 5w40.

“Put simply, the viscosity of an oil gives you an indication of its resistance to flow at given temperatures,” Wright explains. “The ‘w’ rating, preceded by a number, gives you an indication of how the oil flows at winter temperatures and the last number indicates flow in summer temperatures.”

However, the viscosity alone isn’t enough to indicate how well an oil will protect your engine. There are numerous tests that car manufacturers go through to formulate engine oils and these are ratified by an organisation called the European Automobile Manufacturers Association, known as the ACEA (an acronym of the organisation’s French title). The ACEA determines the exact requirements needed to meet the demands of different types of modern engine. The specifications, known as ACEA Oil Sequences, are renewed every four years but are backwards compatible for older vehicles. 

“The high costs of investment in engine oil technology run into six or even seven figures for a set of engine tests. Oil blenders, manufacturers and marketers have to be certain that the lubricants they sell are suitable for a given application,” Wright says.

Each ACEA specification is denoted by a letter, followed by a number, which identifies the class of oil and a category within that class – for example, ACEA C3. This specification will be indicated on the oil packaging and the car manufacturer's manual will again specify which you should go for.

But what if you can’t find the exact recommended oil? Wright says drivers should tread carefully, because using the wrong oil can cause damage to your car’s engine.

The problem is that the oil industry is self-certifying. The ACEA defines the specifications, but it’s up to the manufacturers to make sure that the products they put out meet them. A technical organisation called ATIAL spot-checks products of its members every few years, but largely it’s down to the industry to self-police. Some lesser known oil suppliers have been known to label their products as a certain specification but not meet the standards.

“Sometimes it distorts the marketplace if you get some less reputable new entrants into a market selling product below the cost price of other, more reputable blenders. That’s when you get some questions raised.

“If something looks too good to be true, then it probably is,” Wright says. “If you have been offered the latest specification, fully synthetic engine oil in the pub for £1 a litre, then it probably is too good to be true.”

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To combat such products and to check the quality of products, the UKLA launched the Verification of Lubricant Specifications (VLS) service. Anyone can suggest a product for it to test, and if it’s found to be wanting, the UKLA will ask the marketer to quarantine or recall the product, and relabel or reformulate it. Wright says that since the VLS was launched in 2013, it has made a positive impact on the quality of lubricants sold in Britain.

“I think we’re almost there,” he says. “Some of the newer companies are marketing strange formulation that need investigating but, on the whole, the UK market is quite compliant.”

Conversely, some manufacturers produce oils that go beyond the required specifications.

“Some manufacturers will blend to the base line and some will look to exceed it, to develop some form of quality positioning around their brand,” Wright explains. “It’s usually the multinationals, the big companies that will make them even better than the specifications. So if an oil specification says it’ll last for 15,000 miles, they’ll make it so it lasts 20,000 miles, for example. They’ll put a richer mix of additives into the formula to make sure that not only does it not produce wear in an engine, but it’ll help protect the engine over time, too.”

Quick questions

How does oil vary?

Oil comes in different viscosities, signified by a grade number, such as 5w40. It’s also blended for different types of engines, signified by an ACEA specification, such as ACEA A1/B1.

What oil suits what car?

The type of oil recommended by the car manufacturer will be listed in the manual. It may recommend a particular brand, but the grade and ACEA specification are the important things to look for.

Does brand matter?

As long as the grade and ACEA specification is correct, you should be fine. However, David Wright of the United Kingdom Lubricant Association recommends choosing reputable manufacturers. Some firms will blend more advanced, expensive oils that exceed the specifications on the packaging.

What about cheaper brands?

Again, as long as the grade and ACEA specifications are correct, then everything should be okay. But oils from some newer, less reputable brands on the market have been found not to meet the claimed specifications. The UKLA’s VLS service investigates complaints and tests products to see if they’re compliant and has an archive of its findings on its website -

What happens if you use the wrong oil?

Using the wrong oil will put undue stress on the mechanical components of your engine. Badly lubricated parts will wear faster and so decrease the life of the engine.

How often should you check your oil?

There’s no set rule. Those who drive more should check more often. David Wright suggests everyone should check their vehicle's oil levels at least once a month.

How often should you change the oil in your car?

This will vary from car to car and will be mentioned in the car’s manual.

Glossary of terms

ACEA - European Automobile Manufacturers Association. An organisation that represents European vehicle makers and determines the requirements for modern lubricants. Specifications for different types of oil are denoted by an ACEA figure on packaging, such as ACEA A1/B1.

ATIEL – Industry association of European lubricant manufacturers and marketers. The acronym comes from the French “Association Technique de l'Industrie Européenne des Lubrifiants”

Grade – the viscosity of your engine oil, signified by two numbers that indicate oil flow in winter and summer temperatures (eg 5w40). The lower the numbers, the thinner the viscosity.

SAE - Society of Automotive Engineers. The US-based body that defines how oil viscosities are defined and regulated.

UKLA – United Kingdom Lubricants Association. Body representing businesses in the British lubricant industry.

VLS – Verification of Lubricant Specifications. A non-profit service launched by the UKLA to investigate complaints of non-compliance with lubricant specifications.

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29 August 2016
I suspect that the majority of readers will rely on the dealer servicing and will have no choice in their car's lubricant. If so, then it's a good idea to purchase your own oil (at a decent discounted price!) and instruct the dealer to use it instead of their own oil (typically supplied by the litre at full retail rate). Also be sure to check the dipstick following a service. Last time I did this, I discovered that my car had been overfilled by about 1.5 litres. I think what happens is that many dealers suck out the old oil via the dipstick tube, then refilled with new oil measured out to the manufacturer's spec. So any residual oil in the sump, or in the oil filter if it has not been replaced, will add to the capacity.
Be aware that overfilling is just as dangerous to your engine's health as underfilling, because both can cause oil starvation.


29 August 2016
....was the title.
I have just read the whole blog to be told read my cars manual!
Great advice....given that the manual doesn't say.

29 August 2016
I always check my oil after a service and on one occasion it hadn't actually been changed at all. Nothing had been done. The air cleaner box was filthy and full of dried leaves. I was shocked, took the car back and was even more shocked at the hostility from the dealer who treated me as some kind of liar despite all the proof being on my side (i.e., a car with oil like back paint and a filthy air filter despite a receipt that charged for both...). How do you even know if they're putting the correct oil in?

30 August 2016
Early days with an Imp, being serviced at a Rootes garage. Opened the coolant tank after coolant replacement to find oil floating on top. Mechanic had used the same jug for oil and coolant. Sadly, I also always check a mistake has not been made!

29 August 2016
i never check it,my Car's Brain does,plus,if i want to check it, i can do it on the move,as for correct type of Oil, i use what the maker or Garage recommend.

Peter Cavellini.

8 June 2017
Engine oil is very important for every car, it helps to smooth the engine, gear, clutch, and riding and also it helps to improve the mileage But it is important to choose the best oil for our car so that we can easily expand the lifespan of our car. According to experts, we have to change our engine oil after every 5k kilometers or less, I would like to follow the instructions from here to recognize the best engine oil.

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