How Often Should You Change Your Engine Oil?

Fresh engine oil is one of the cheapest and most effective forms of protection you can give your engine. It lubricates, cools, and carries harmful contaminants away from precision components. But it does not last forever, and leaving old oil in too long is a false economy that can lead to expensive damage. Here is how often you should change it, which grade your car needs, and what to watch out for between services.

How Often Should the Oil Be Changed?

The right interval depends on your car and the type of oil it uses:

• Standard mineral or semi-synthetic oil: Every 6,000 to 10,000 miles or every 12 months, whichever comes first. This applies to most older vehicles (roughly pre-2010).
• Full synthetic oil (most modern engines): Every 10,000 to 20,000 miles or every 12 months. Many newer petrol and diesel engines with variable service intervals run full synthetic and can go further between changes, but most independent experts still recommend an annual change even if the on-board reminder has not kicked in yet.

The manufacturer's recommended interval in the vehicle handbook should always be your primary reference. The service reminder on modern cars factors in driving style, short trips, and engine temperature, which makes it useful but not something to rely on blindly.

What Goes Wrong If You Leave It Too Long?

Oil that stays in the engine past its sell-by date degrades in several ways:

• The additive package, which provides anti-wear, cleaning, and dispersant properties, gets used up.
• Combustion by-products and tiny metal particles build up, turning the oil into an abrasive rather than a protector.
• Oxidation thickens the oil and creates sludge, which can block the narrow oil passages feeding the camshaft and valvetrain.

The long-term cost of habitually stretching oil change intervals includes faster engine wear, sludge accumulation (which is expensive to clean out), and in serious cases premature engine failure. Engines with variable valve timing (VVT) are especially sensitive to oil quality and flow.

Which Oil Does Your Car Need?

Engine oil is defined by its viscosity grade (such as 5W-30, 5W-40, or 0W-20) and its performance classification (such as ACEA C3 or API SN). Using the right spec matters because the wrong oil can hurt fuel economy, interfere with emissions system performance, and in some cases damage components.

You can find your car's required oil specification:

• In the vehicle handbook, usually in the technical data section towards the back
• On a sticker under the bonnet or on the oil filler cap on some models
• Via the manufacturer's website using your registration number

Your garage will use the correct spec as standard, but it is worth knowing if you are topping up at home. Putting in a thicker oil than specified can reduce fuel economy and make cold starts harder than they need to be.

Topping Up Between Services

It is normal for engines to use a small amount of oil between services, particularly as they accumulate mileage. Most manufacturers consider consumption of up to 1 litre per 1,000 miles acceptable, though many modern engines burn far less than that.

Check the oil level at least once a month and before any long drive. Pull out the dipstick, wipe it clean, push it all the way back in, then pull it out again and read the level. It should sit between the MIN and MAX marks. If it is at or below MIN, add the correct-specification oil in small amounts, checking after each addition. Do not go above the MAX mark. Overfilling can cause the oil to aerate and foam, which reduces its ability to lubricate, and excess oil can damage seals.

If the level drops noticeably between services, or you spot blue smoke coming from the exhaust (a sign that oil is burning inside the combustion chamber), get the engine checked at a garage.