Expert advice: what you should know about checking engine oil

If the engine is the heart of your car, the oil is its blood, but checking engine oil is a lot simpler than major surgery! Without oil your engine can’t function as the oil lubricates all the moving parts and ensures your engine leads a long, healthy and happy life. It’s a worry that surveys show the majority of drivers can’t and don’t check the oil level in their cars because if the lubricant level gets too low, an engine will literally grind to a halt. 

That doesn’t just mean Green Flag’s technicians will have to come and rescue drivers whose engines have run out of oil; it may also lead to major and very expensive repairs to engines, and in some cases new engines altogether. But checking, and if necessary, topping up the oil couldn’t be easier.

Checking engine oil: how frequently to check it

Most cars have an oil warning light that should glow red if the oil level gets too low. However, if you leave it until the red light comes on, the engine may have suffered seriously. Ideally, we’d recommend drivers check their oil weekly. If you can’t manage that, at least do it monthly.

Checking engine oil: when to check it

Ideally the car will be parked on a flat surface and the engine will be cold. If you want to check the oil after you’ve been driving the car, wait for at least 15 minutes. This doesn’t just let all the under bonnet bits cool down, it also allows all the oil to drain back into the sump so you can get an accurate gauge of how much you’ve got.

Checking engine oil: find the dipstick

Some cars have electronic dipsticks that do the measuring for drivers, saving them from getting their hands dirty, and are accessed through the car’s on-board computer. The majority have the physical item that you’ll find beneath the bonnet. The dipstick is exactly what its name suggests: a stick that dips into the oil in the engine’s reservoir or sump. The dipstick generally has a loop at the top that you hook your finger round to pull it out. In many cars the loop will be of yellow plastic.

Checking engine oil: check the level

First pull out the dipstick, then use a cloth to wipe it clean. You want to use a cloth made from a lint-free material like the cotton of an old T-shirt so that it doesn’t leave any bits behind. Once you’ve wiped the dipstick, push it all the way back in, then withdraw it again. At the bottom of the dipstick there will be markers showing the minimum and maximum points and the residue oil on it should be somewhere between the two.

Checking engine oil: Which sort should you buy?

You might think oil is oil but the viscosity (how effectively it lubricates) can vary greatly. Your car’s handbook will tell you exactly what to buy. If the car doesn’t have a handbook, ask a garage you trust or call the main dealer. Oils have different ratings denoted by the Society of Automotive Engineers (SAE). Most cars will use multi-grade oil. This is designed to lubricate equally in extremes of weather from very cold to very warm. A popular SAE rating is 10W-30. This is an oil that can be used in the winter (hence the W), and has a viscosity rating of 10 in cold weather and 30 in warm weather.

Checking engine oil: What does the filler cap look like?

Most oil filler caps are black and located on the top of the engine, usually in the middle of the engine bay. If you can’t spot it, check the car’s handbook.

Checking engine oil: How to fill it

Remove the oil filler cap and put it somewhere safe where it won’t get dirty or fall into the engine bay. It’s worth investing in a plastic funnel or small pouring jug for this. Pour 100ml of oil into the open oil tank. Wait a few moments, then go through the checking process above.

Checking engine oil: How much to put in?

Depending on how much or little oil the car had in it, 100ml may be all it needs. But keep adding oil in small amounts until the level on the dipstick sits between the minimum and maximum markers.

Checking engine oil: Don’t put too much in

Believe it or not, we’ve had technicians called out because cars have too much oil in them. This causes a range of problems as it can prevent the engine breather system working properly. The result can be anything from a large build-up of a mayonnaise-like substance by the oil filler cap to severe engine damage as a result of the excessive oil entering the combustion chamber. The latter can be a very costly repair. On petrol engines the catalytic converter can be damaged and lead to your vehicle failing its MOT. On diesel engines it can prompt a phenomenon called ‘dieseling’ where the engine runs on its own oil, uses it all up very quickly and then explodes. If in doubt ask your local garage to help you check your oil level.

Nick Reid is a fellow of the Institute of the Motor Industry and head of automotive technology at Direct Line Group