When to change engine oil: mileage is not the only guide

What the manufacturer says

Most petrol cars have a normal interval of 10,000–15,000 km or one year. Next to it there is almost always a second, “severe” schedule, usually one and a half to two times shorter. It covers short trips, traffic, dusty roads, heat, towing and taxi work.

Go through that list honestly and most city cars turn out to be on the severe schedule. Manufacturers don’t treat this as unusual; they just describe it separately.

Engine hours: why mileage misleads

Oil ages with running time, temperature and combustion by-products, not with kilometres as such. Average speed tells you how many hours of running sit behind the mileage.

Over 10,000 km on the highway the engine runs for about 125 hours; in traffic, four times as long. If your trip computer shows average speed, it tells you which schedule your car is closer to.

Cars running on CNG or LPG

Gas burns hotter than petrol. The oil collects less soot, but it oxidises faster, which is why many mechanics shorten the interval for cars with gas equipment. If your car runs on gas most of the time, use the severe schedule and check the level more often.

What the dipstick shows

Check the level every week or two and before a long trip: on level ground, 5–10 minutes after switching off a warm engine. It should be between the marks. If you add more than half a litre per 1,000 km, have the car checked.

Colour says little about when to change. Modern oil darkens quickly because its detergents hold soot in suspension; that is the oil doing its job. These are the real warning signs:

• the oil looks like milky coffee or there is emulsion on the dipstick: coolant is getting into the oil;
• it smells strongly of petrol: fuel is thinning the oil, usually because of short trips;
• the oil pressure warning light comes on: stop and switch off the engine straight away; do not drive to a workshop.

At every oil change

• Change the oil filter together with the oil.
• Fill the amount given in the manual and check the dipstick afterwards.
• Note the date and mileage so you notice if the car starts using oil.
• In the dusty season, check the air filter too: dust that gets past it acts as an abrasive in the oil.