Engine oil keeps hundreds of fast-moving metal parts inside your engine from grinding against each other. Over time it breaks down with heat, collects dirt and soot, and slowly stops protecting the engine. A regular engine oil change is the single most effective thing you can do for your car’s long-term health, and one of the cheapest.

This guide explains what engine oil actually does, what happens step by step during an oil change, how to choose the right oil, and what decides the cost, so you know exactly what you’re paying for.

What engine oil does

• Lubricates moving parts like pistons, bearings and the camshaft so they slide on a thin film of oil instead of rubbing metal on metal.
• Cools the engine by carrying heat away from the hottest parts, working alongside the coolant.
• Cleans by holding soot, carbon and tiny metal particles in suspension until the oil filter catches them.
• Seals the small gaps between the piston rings and cylinder walls, which helps the engine keep its power.
• Protects against corrosion with additives that neutralise acids formed during combustion.

Old oil does all of these jobs badly. The result is more wear, more heat, more noise and lower mileage, and in the worst case, serious engine damage.

What happens during an engine oil change

1. The engine is warmed slightly so the old oil flows out easily and carries more dirt with it.
2. The car is raised and the drain plug under the oil sump is opened. The old oil drains out completely.
3. The old oil filter is removed and a new one is fitted. Fresh oil with an old, clogged filter defeats the purpose.
4. The drain plug is refitted and tightened, with a new washer where needed to stop leaks.
5. Fresh oil of the correct grade and quantity is poured in through the filler cap.
6. The engine is run for a minute, switched off, and the level is checked on the dipstick and topped up if needed.
7. The service reminder is reset, if your car has one, and the drain area is checked for leaks.

At a workshop this takes under an hour, and it’s normally part of a periodic car service, where filters, brakes and fluids are checked at the same time.

Which engine oil does my car need?

Your owner’s manual lists the recommended oil grade, such as 0W-20, 5W-30 or 10W-40. The first number, before the W (for winter), shows how easily the oil flows when the engine is cold. The second number shows how thick it stays at normal operating temperature. Modern engines with tight tolerances are designed for specific grades, and using the wrong one can affect protection and fuel economy. Stick to what your car maker recommends.

Mineral, semi-synthetic or fully synthetic?

• Mineral oil is refined from crude oil. It’s the cheapest and has the shortest change interval. Mostly used in older engines.
• Semi-synthetic oil blends mineral and synthetic oils. It lasts longer and protects better under heat, a good middle ground for many cars.
• Fully synthetic oil offers the best protection, flows best on cold starts and lasts longest. Many modern petrol, diesel and turbocharged engines require it.

For how long each type lasts, read our guide on when to change engine oil.

What decides the cost of an engine oil change?

• Oil type and brand: fully synthetic costs more per litre than mineral oil.
• Engine size: small hatchbacks need around three to four litres; bigger engines need more.
• Oil filter: prices vary by model. We recommend original filters.
• Petrol or diesel: diesel engines often need oil made for diesel, and sometimes more of it.
• Labour: usually small, and often included when done as part of a service.

The cheapest oil change is rarely the best value. Correct-grade oil and an original filter cost a little more, but they protect an engine that is far more expensive to repair. For the full picture, see what goes into car service cost.

Should you change engine oil yourself?

It’s possible, but it means safely raising the car, catching four or more litres of hot oil, disposing of it responsibly and fitting the filter correctly. A loose drain plug or a badly seated filter can drain the engine on the road. For most owners, the small labour cost at a workshop is worth it, especially when it comes with a full inspection.

Common questions

Related reading

• When to change engine oil
• Car overheating: what to do
• Swift car service cost and schedule