What's the best oil for my car?
How to choose the best engine oil and the correct grade of oil for your car
An engine will quickly seize up and no longer work if it doesn’t have any oil, and won’t perform to its best if the oil is old and needs changing. When your car has a service, the oil will usually be changed to keep the engine running as well as possible. Sometimes, you might need to add more oil at other points during the year, and the correct oil needs to be put in. The grade and type of oil that your car requires will be written in the car’s handbook.
What are oil grades?
Oil grades refer to the viscosity – or thickness – of the fluid, and there’s a sometimes-confusing array of grades available. To tell which is which, that version will be written on the bottle, shown as four numbers with a letter ‘W’ and dash in between, like 10W-30 or 5W-40.
Modern engines are engineered and tested to a much finer level than was the case previously, and because of that they need much thinner grades of oil than older engines did.
You might have heard the term ‘multigrade oil’, which means the oil has additives to enable it to change how thick it is as the engine heats up. Most modern oils are multigrade and therefore have two different numbers.
The first grade will have the letter ‘W’, as mentioned above. In both cases, the ‘W’ stands for ‘winter’ and shows how cold weather will affect the performance of the oil. When the temperatures are warmer, the oil will perform differently, which is what the second number is for. Once again, your handbook is the place to go to find what oil you’ll need to use.
Different oils have varying specifications as well as types and grades. Petrol-powered vehicles use ‘ACEA’ specifications, showing the performance of the oil. Most European and Asian carmakers display the following ACEA specifications:
A1 Fuel economy petrolA3 High performance and/or extended drainA5 Fuel economy petrol with extended-drain capability
Some oils are developed to withstand up to 18,000 miles or two years of driving before they need changing. Called ‘longlife,’ they enable carmakers to offer longer service intervals.
What is the best oil for a diesel car?
Diesel engines need different oil from petrol engines, especially if they have a diesel particulate filter (DPF) fitted to them.
Like the oil for petrol engines, diesel engine oil is offered in a range of ratings. Your handbook will tell you which is suitable for your car:
B1 Fuel economy dieselB3 High performance and/or extended drainB4 For direct-injection passenger-car diesel enginesB5 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 DPF. SAPS stands for Sulphated Ash, Phosphorous and Sulphur – all substances that can build up in the DPF.
Look for the following oils:
C1 Low SAPS (0.5% ash) fuel-efficientC2 Mid SAPS (0.8% ash) fuel-efficient, performanceC3 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.
What is synthetic oil?
Synthetic oils are more sophisticated than standard mineral oils, and more expensive. Your car’s handbook will tell you if it’s necessary to use synthetic oil.
Fully synthetic oils are the most expensive but semi-synthetic, which combine synthetic and mineral oils, are more widely used.
Do I need to change the oil filter?
It’s best to change the oil filter when you change the oil, since you don’t want any old impurities getting back into your engine. It’ll be changed, along with the oil, at each service.
Check your car’s handbook or with your dealer how often the oil and filter should be changed. Fortunately, you don’t need to change the filter if you’re simply topping up the oil between services.