Our handy guide with help you pick the best engine oil and the correct grade of oil for your car
Oil is a vital substance for the smooth running of your engine, and if you want the engine to have a long life then looking after the oil is fundamental. Naturally, this means making sure that oil changes are performed on time with the most suitable oil for your car. Your local garage should be able to advise you on the best oil to use, but you may run the risk of causing long-term damage to your car’s engine if you don’t make regular dipstick checks between services yourself.
It’s straightforward enough to check the engine oil level, but if you’ve never done it before it’s best to follow the instructions in your car’s handbook. This will provide details of how warm the engine should be, and how long you should wait after turning off the ignition to get the most accurate measurement on the dipstick. But as a general rule, you’re usually good to take a dipstick reading a few minutes after switching off a hot engine, and for most drivers the optimum oil level isn’t an exact science.
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If you do find your oil needs topping up, you’ll find there’s a potentially bewildering range of options at your local garage or car accessory store. For example, the best oil for diesel engines will be entirely different to the best oil for petrol engines, while high performance oils are available for turbocharged engine. Another confusing factor can be labels for synthetic oils and semi-synthetic oil. There are a multitude of different grades to choose from too, and that’s why we’ve put together this guide to choosing the best oil for your car.
What are grades of oil?
There’s an overwhelming range of oil out there. Not only are there dozens of brands – some you’ll have heard of, others you may not – but they all come in a range of different grades.
If you look at the label you’ll see a number of digits, such as 10W-40 or 5W-30 – and it’s these numbers that you’ll need to get right (see below).
Those numbers refer to the viscosity – or ‘thickness’ – of the oil. Today’s oil is generally thinner, allowing the oil to flow around the engine quickly when the car is started, which helps to prevent damage caused by metal parts grinding against one another. Modern engines are built with finer tolerances and therefore require thinner oils. Thinner oils – those with lower viscosity – also help with economy.
Most oils carry two sets of numbers because they’re ‘multigrade’ oil. Additives in the oil mean it can change viscosity depending on temperature.
The lower the first number, the better the oil will operate at low temperatures – hence the ‘W’ digit, which denotes winter. The lower the second number, the better it’ll operate at higher temperatures.
You can also mix different oil brands and even different grades without severely damaging your engine. Having the wrong oil in your car engine rather than no oil is the lesser of two evils, albeit if you find yourself stuck and only able to access the wrong grade of oil, you should book your car for an oil change as soon as possible.
To complicate matters, there is a range of different engine oil standards too, but it’s the ACEA numbers on each oil container that are most relevant.
European, and many Asian carmakers generally use the following specifications for petrol engines. Specifications for diesel engines are covered separately below.
A1 fuel economy petrol
A3 High performance and/or extended drain
A5 Fuel economy petrol with extended drain capability
How do I find the correct grade of oil for my car?
The two best ways to find out the correct grade of oil for your car are to take a look in your car’s owner’s manual, or to give your local main dealer a call.
By giving your main dealer your vehicle’s registration number, they’ll be able to look your car up on their system and tell you the correct grade in seconds.
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Take a look at the oil container to ensure the information matches up. It’s worth noting that some carmakers use their own oil specifications, and may list an acceptable alternative grade or specification that’s more widely available. This information will be listed in your car’s handbook.
If you have any problems, you should call your local main dealer for advice.
What is synthetic oil?
Some modern engines require synthetic oils, which contain fewer impurities. Whether you can use them will vary from engine to engine, so take a look at your car’s handbook or call your local dealer to check.
There are two main types. Fully synthetic oils offer what is regarded to be the highest performance for modern engines. Semi-synthetic oils contain a mix of synthetic and mineral oil. Synthetic oils are usually more expensive than standard oil, but that extra cost can be offset by the lower frequency at which they need to be changed.
When do I need to change my oil?
Some clear indicators that your car needs an oil change include; dark and dirty oil, loud engine noise and knocking, oily smells in the cabin and most obviously, if the oil change or check engine light is on.
Some cars with worn engines burn through a lot of oil – a smoky exhaust can be a sign of this – and it’s not uncommon for cars to develop oil leaks too. If your oil level is dropping and needs topping up, you’ll require a trip to your garage for professional advice, but not before you’ve topped the oil up.
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You can also get ‘longlife’ oils, which are usually fully synthetic. These can last for up to two years or 18,000 miles between changes, saving you money by having longer service intervals. Always check whether your car is compliant before choosing the longlife oil option first.
Do I need to change my oil filter?
When you have a full oil change, you must change your oil filter – it’s an essential part of a service.
The filter retains a small amount of oil, meaning that your new, clean oil will be contaminated with old, dirty oil if you don’t change it. That’s important because the primary reason for having your car’s engine oil changed is to remove those contaminants.
It’s less important to change your oil filter if you’re simply topping up the oil levels, but you should always ensure your oil filter is changed whenever your car is serviced. How often and how many miles can pass between oil and oil filter changes varies from car to car. Check your owner’s manual or with your local dealer for more information.
What is the best oil for my diesel car?
Diesel engines have very different requirements to petrol engines, so you must ensure you use the correct oil for your car. It’s particularly important if your car is fitted with a diesel particulate filter (DPF) as anyone found driving a diesel vehicle without a DPF could be fined up to £2,500.
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As with oil for petrol engines, diesel-specific oil has a range of ratings, so check your handbook for the most correct one.
Diesel engine oil:
B1 Fuel economy diesel
B3 High performance and/or extended drain
B4 For direct injection passenger car diesel engines
B5 Fuel economy diesel with extended drain capability
If your car has a diesel particulate filter, you must use a ‘low SAPS’ oils or you risk blocking the DFP. SAPS stands for Sulphated Ash, Phosphorus and Sulphur – all substances which can build up in the DPF.
Look for the following oils:
C1 Low SAPS (0.5% ash) fuel efficient
C2 Mid SAPS (0.8% ash) fuel efficient, performance
C3 Mid SAPS (0.8% ash) less fuel efficient; more bias on performance
Each carmaker will choose the SAPS level most effective for their engine design and so will choose a different ACEA C rating as appropriate.
Let us know if you’ve encountered any oil-related horror stories in the comments below and check our guide to fuel additives here…