← All guides

Maintenance and tyres , Costs and money , Know your car · Updated 2026-10-18

Engine oil explained: what it does and how to check it

Engine oil is the lifeblood of the engine. It reduces friction between moving parts, carries heat away, keeps the inside clean and helps protect against rust. Running an engine with too little or the wrong oil is one of the quickest ways to cause expensive damage, so it is worth understanding the basics.

What oil actually does

• Lubricates: a thin film keeps metal parts from touching each other.
• Cools: it carries heat from places the coolant cannot reach.
• Cleans: it holds soot and tiny particles until the filter or the next oil change removes them.
• Protects: additives fight corrosion and foaming.

Over time oil wears out. It collects dirt, fuel and moisture, and the additives are used up. That is why oil and the oil filter are replaced on a schedule, covered in how often to service.

Reading the grade

A bottle shows a code such as a number, the letter W, and another number. The first number with W describes how well the oil flows when cold, and the second describes how thick it is at working temperature. You do not need to understand the chemistry. What matters is that you use the grade the maker recommends for your engine.

Besides the viscosity grade, the bottle lists approvals and specifications. Some engines, especially diesels with particulate filters and modern petrol engines, need oil of a specific type. The correct grade and specification are written in the owner's manual. If a workshop proposes something different, ask why.

Mineral, semi-synthetic and synthetic

Synthetic oils are made to stay stable over a wider range of temperatures and usually last longer. Mineral oils are simpler and older. Most modern engines are designed for synthetic or semi-synthetic oil, and an older engine may be fine with what its manual says. Do not change type just because someone said it is better. Follow the manual, and remember that an engine that has been well maintained all its life should not be switched casually.

How to check the level

This is a safe, simple job that every owner should know. A few minutes a month is enough, and also before a long trip.

1. Park on level ground and switch the engine off. Wait several minutes so the oil can drain back to the pan.
2. Open the bonnet, find the dipstick, pull it out and wipe it clean with a cloth.
3. Push it fully back in, then pull it out again.
4. Read the level. It should lie between the minimum and maximum marks.

Be careful: parts of a recently running engine are hot. Some newer cars have no dipstick and show the level on the dashboard instead. Follow the manual for those.

Topping up

If the level is near the minimum mark, add a little oil of the correct grade through the filler cap, wait a minute, and check again. Add small amounts at a time. Too much oil is also harmful, because it can foam and put stress on seals. If you need to add oil often, something is wrong and the car should be inspected.

What the oil can tell you

• Dark oil is normal on a used engine, especially a diesel. Colour alone is a poor guide to quality.
• Milky or creamy oil may mean coolant is mixing in. Do not drive the car and have it checked.
• A smell of fuel or a level that seems to rise can mean fuel is entering the oil.
• Metal glitter on the dipstick points to internal wear.
• Falling level means the engine burns or leaks oil. Look for drips under the car.

The oil warning light

A red oil can symbol means pressure is too low, and the engine should be stopped as soon as it is safe. Do not keep driving to "make it home". Check the level, and if the light stays on or you are unsure, call for help. A different, yellow oil symbol on some cars may refer to the level or to oil condition; check dashboard warning lights and your manual.

Oil when buying a used car

Look at the oil and the filler cap when you inspect a car, and ask for evidence of regular oil changes. The tips in how to inspect a used car cover what else to look for under the bonnet.

What to remember

• Use the grade and specification written in the manual.
• Check the level regularly, on level ground with the engine off for a few minutes.
• Add only small amounts, and never overfill.
• Milky oil, fuel smell or a falling level need a workshop visit.
• A red oil light means stop the engine safely.

Beginner Checklist Do it yourself Engine Oil Service history