Which engine oil does my car need and how do I top it up?

Reading the grade and the manufacturer specification your engine actually needs, checking the level properly, and topping up without overfilling.

Difficulty: beginner
Time: 20 min
Read: 4 min
Safety: caution

Short answer

Your handbook states both a viscosity grade such as 5W-30 and a manufacturer specification code — the specification matters more than the grade. Check the level on level ground with a cold engine, and top up in small amounts, rechecking between each, until the level sits between the two dipstick marks.

Modern engines are fussier about oil than older ones because the oil has to work with the emissions equipment as well as lubricate. Two oils of the same grade can be entirely different products, and the wrong one can block a diesel particulate filter.

Safety

Let the engine cool before opening the filler cap or touching anything under the bonnet — exhaust components stay hot enough to burn for a long time after a drive. Never overfill: excess oil is churned by the crankshaft and can damage the engine and the catalytic converter. Used oil is harmful to skin and must be taken to a recycling centre, never poured down a drain.

What you'll need

The correct oil for your car
A funnel
A rag or paper towel

Where the checkable reservoirs sit under a bonnet

Step by step

Find the specification in your handbook.You are looking for two things: a viscosity grade such as 0W-20 or 5W-30, and a specification code such as an ACEA class or a manufacturer approval. The approval code is the one that matters most on modern engines.
Buy oil that lists that approval on the bottle.Manufacturer approvals are printed on the back of the container. If the exact approval is not listed, it is not the right oil, whatever the grade on the front says.
Park level and let the engine cool.A slope gives a false reading and a hot engine has oil still draining back into the sump. Leave it at least ten minutes after switching off, longer if you can.
Pull the dipstick, wipe it, and put it fully back.The first reading is always contaminated by oil splashed up the tube. The second reading, after wiping and full reinsertion, is the real one.
Read where the film ends.It should sit between the two marks, ideally nearer the upper one. The gap between them is usually around a litre, so a level halfway up means about half a litre low.
Add in small amounts through the filler cap.The cap on top of the engine, usually marked with an oil can symbol. Add perhaps a third of a litre, wait a minute for it to drain down, then recheck. This is how you avoid overfilling.
Stop at the upper mark, not above it.Overfilling is genuinely damaging. If you overshoot, it needs draining rather than driving off and hoping.
Check some cars have no dipstick.Many newer cars use an electronic level readout in the dashboard menus instead. The handbook explains the procedure, which usually requires the engine warm and idling on level ground.
Note how much you added and how often.Some engines use oil normally. An engine that suddenly starts using much more, or that leaves a puddle, needs a garage rather than a bigger bottle.
Take the empty container and any old oil to a recycling centre.Household waste recycling centres take engine oil. It is illegal and seriously polluting to put it down a drain.

Tips

Keep a one-litre bottle of the right oil in the boot. It is the difference between a top-up and a recovery truck.
Photograph the handbook page with the specification on it — it is the thing you will need in a shop and never have with you.
A dark oil is not necessarily a worn-out oil. Diesels blacken oil almost immediately and it is normal.

Common mistakes

Buying by viscosity grade alone — Two 5W-30 oils can meet completely different specifications. On a car with a particulate filter, a full-ash oil in place of a low-ash one shortens the filter's life considerably.
Filling to the top mark in one go — The gap between marks is smaller than most people assume, often around a litre. Pouring in a whole bottle from halfway usually overfills it.

If it doesn't work

Oil level drops noticeably between checks

Cause: Consumption or a leak — Fix: Look for drips where the car parks. Either way, have a garage look at it rather than simply topping up indefinitely.

Oil warning light comes on while driving

Cause: Low oil pressure, which is not the same as low level — Fix: Stop as soon as it is safe, switch off and do not restart. This light is a red one for a reason and driving on can destroy the engine.

Overfilled the engine

Cause: Topped up too much at once — Fix: Do not drive it. Have the excess drained — a garage will do this quickly and it is far cheaper than the alternative.

Milky deposit under the oil filler cap

Cause: Condensation from short journeys, or something more serious — Fix: Common on cars used only for short trips. If it persists after a long run, or the coolant is also dropping, get it checked.

Questions people ask

Can I mix different oils?

In an emergency, topping up with a similar oil is better than running low. But mixing specifications regularly is not a good idea, and the next service should return the engine to the correct oil throughout.

What does 5W-30 mean?

The number before the W describes how the oil flows when cold, and the number after describes it at operating temperature. A lower first number flows better on a cold morning, which is when most engine wear happens.

How often should I check the oil?

Monthly, and before any long journey. Some modern engines legitimately use oil between services, and the only way to know yours does is to look.