There are more car engine oils available on the market than probably any other lubricating oil product. It can get very confusing if you don’t understand the differences between the products, or know which one is right for your car. Many people are guilty of sticking with the brand and oil that they know, or just buying the cheapest in the garage or store. What you may not realise, is that by using the wrong engine oil in your car, you could cause lasting and expensive damage.

With so many oils available on the market, where do you start? The easy answer is your vehicle manual. That will tell you what viscosity or grade of oil is recommended for your car, for example a 5W-30 engine oil. You may have seen oils referred to by an SAE (Society of Automotive Engineers) number, but that also refers to viscosity.

If you’ve ever wondered what the ‘W’ stands for in the engine oil grade, it’s Winter. Lubricating oil thins as it heats up and thickens as it cools down, so it’s important to get the correct viscosity for the average temperatures your engine is operating in, to make sure your engine is well protected. The number preceding the W gives an indication of the oil’s performance at lower temperatures. Typically you will find 5W, 10W or 15W oils available, but newer formulations now perform better at low temperatures and meet the 0W rating. If your oil is too thick and you live in a cold environment, you are likely to experience problems with start-up, and the engine may not be properly protected until the oil has warmed up. The second number in the viscosity refers to the oil’s performance once it has heated up. A 40 viscosity oil will be thicker at high temperature than a 30. It is essential to, as a minimum, refer to the manufacturer’s manual for an oil recommendation so that you select the right viscosity. If your engine oil is too thin, it can cause excessive engine wear. If it is too thick, it can cause oil starvation to components and increase drag, which in turn reduces fuel efficiency. Both cases can lead to expensive repairs to the engine.

You should consider whether your vehicle has a petrol or a diesel engine, as they operate differently, so may require different oils and additives. There are a number of oils in the market which are blended specifically for petrol or diesel engines, and they should state this on the packaging.

Many modern engines are engineered to very high specifications, and provide very high levels of performance. It’s essential that the oil selected for these cars is capable of protecting the engine, especially at the higher end of their performance capabilities where engines are running fast and hot.

If you cover considerable miles in your car, you will want to think about the frequency your oil needs changed. Many of us only change the oil when our car is serviced, but if you’re covering a lot of miles, you may need to top the oil up now and again. The two main things to consider here are oil compatibility and quality. If you are topping up your engine oil, make sure that you use the same product or another one which is compatible. Mixing incompatible oils can lead to engine wear, sludge and even engine failure. It’s usually the case with engine oils that you get what you pay for. More expensive oils are blended with additive packages which increase the protection they give to the engine for a longer period.

When it comes to synthetic versus mineral oil, it’s best to refer to the manual or the manufacturer’s engine specification. In many older cars, synthetic oils might not offer significant benefits. However, in modern engines, the benefits and higher quality offered by synthetic lubricants can have an impact on engine cleanliness, engine protection and fuel efficiency.

If you’re still unsure, many oil manufacturers have tools on their website where you can enter your car registration number or make and model, and they will tell you which of their oils meets the right specifications. Examples include Mobil, Shell, Castrol and Fuchs. Be aware that only certain oils will be listed as ‘recommended or approved’. This means that the vehicle manufacturer has recommended or approved the oil for that specific engine. The majority of oils will be listed as ‘meeting the specifications of’, which means that the product has been developed to meet the manufacturer specifications, but has not been endorsed by them.

So before you top up your new car using that old bottle of generic 15W-40 oil that’s been in the garage for a very long time, stop to think about the potential damage it might cause, and grab your manual!