What’s the best oil for my car?

“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.”

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 - http://www.ukla-vls.org.uk.

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.