Engine oil is the lifeblood of your car's engine. It lubricates moving parts, dissipates heat, and carries contaminants away from critical components. Over time it degrades and loses its protective properties — making regular oil changes one of the most important and cost-effective things you can do to extend engine life. This guide explains how often to change oil, what grade to use, and what happens if you leave it too long.
Recommended Oil Change Intervals
The correct oil change interval depends on your car and the type of oil it uses:
- Standard mineral or semi-synthetic oil: Every 6,000–10,000 miles or every 12 months, whichever comes first. Most older vehicles (pre-2010) fall into this category.
- Full synthetic oil (modern engines): Every 10,000–20,000 miles or every 12 months. Many modern petrol and diesel engines with variable service intervals use full synthetic oil and can go further between changes, but annual changes are still recommended by most independent experts even if the on-board service reminder has not yet triggered.
Always follow the manufacturer's recommended interval in the vehicle handbook as the primary guide. The on-board service reminder on modern cars takes into account driving style, short journeys, and engine temperature — making it a useful but not infallible indicator.
What Happens If You Leave It Too Long?
Oil that is not changed on schedule degrades progressively:
- The oil's additive package — which provides anti-wear, detergent, and dispersant properties — depletes.
- Combustion by-products and metal particles accumulate in the oil, increasing its abrasive content.
- Oxidation causes the oil to thicken and form sludge, which can block narrow oil galleries to the camshaft and valvetrain.
The long-term results of consistently extended oil change intervals include accelerated engine wear, sludge build-up (which is expensive to clean out), and in severe cases premature engine failure. Engines with variable valve timing mechanisms (VVT) are particularly sensitive to oil quality and flow.
Which Engine Oil Does My Car Need?
Engine oil is specified by its viscosity grade (e.g. 5W-30, 5W-40, 0W-20) and its performance classification (e.g. ACEA C3, API SN). Using the correct specification is important — the wrong oil can affect fuel economy, emissions system performance, and in some cases damage components.
Find your car's required oil specification:
- In the vehicle handbook (oil specification section, usually near the back)
- On a sticker under the bonnet or on the oil filler cap on some vehicles
- Via the manufacturer's website using your registration number
Your garage will use the correct specification as a matter of course, but it is worth checking if you are topping up at home. Using the wrong viscosity grade — particularly a thicker oil than specified — can harm fuel economy and make cold starting harder.
Topping Up Between Services
Most engines consume a small amount of oil between services, particularly at higher mileages. A consumption rate of up to 1 litre per 1,000 miles is considered acceptable by most manufacturers, though many modern engines consume significantly less than this.
Check your oil level at least once a month and before any long journey. Pull out the dipstick, wipe it clean, reinsert fully, then withdraw and read the level — it should be between the MIN and MAX marks. If the level is at or below MIN, add the correct-specification oil in small increments, checking the level after each addition. Do not overfill beyond the MAX mark — excess oil can aerate and foam, reducing lubrication, and can damage seals.
If your oil level drops significantly between services, or if you notice blue smoke from the exhaust (indicating oil burning in the combustion chamber), have the engine inspected at a garage.