There are so many different makes, types and grades of engine oil that buying a new one can be a daunting process for car owners. Here we demystify choosing an oil and explain some of the terms you’ll come across.

Which engine oil?

Go to your car’s user manual. Here you’ll find out what the car’s maker recommends for your motor. If you don’t have your car’s user manual at hand, or can’t find the relevant page, access the internet. All major oil manufacturers such as Shell or Castrol and even retailers such as Halfords have handy tools for finding the right oil. All you do is type in your car’s registration number and it’ll tell you what oil you need.

What do the numbers on an oil mean?

When you see an oil for your car it will have two numbers and a W in between them. This is its viscosity and shows how suitable an oil might be for your engine and the climate you live in.

Viscosity refers to how easily a fluid flows. The hotter oil becomes the thinner it gets (less viscous). As it cools it becomes thicker (more viscous). The first number is the oil’s viscosity in cold weather during winter – hence the W. The second viscosity number is for normal operating temperatures. Hence a 10W-40 oil flows more easily when hot and cold than a 20W-50 oil.

Conventional or synthetic oil?

This really depends on the kind of car you drive. If it’s an older model, it probably won’t derive much benefit from synthetic oil. Newer cars (after around 2000) will because they operate at much higher pressures and to much more exacting tolerances.

Conventional and synthetic oil start life as the crude oil we hear about when fuel prices are about to rise. And both are refined. The difference is companies go to much greater lengths to remove the impurities from synthetic oils than with conventional mineral oils. They even tailor individual molecules to suit particular engines.

By messing about with the oil’s chemical composition, scientists can develop oils that flow much more freely; that course through the engine the moment the key is turned; and that enhance performance and protection.

Any oil company will advise you to use synthetic oils as they’re better for the majority of engines. But they’re expensive. And that’s why fuel companies have developed semi-synthetic oils. These have many of the benefits of synthetic oil but they’re more affordable.

Does the brand matter

Do you swear by Heinz beans or go for a supermarket’s own brand? With oil, the difference between large well-known specialist brands and names you’ve never heard of is that the Shells and Castrols of this world spend millions every year developing their oils to be as efficient as possible. In addition, they work with car manufacturers to develop oils to suit specific engines, hence why your car’s user manual will recommend a particular brand and grade of oil.