How often should you change engine oil?
Manufacturer intervals assume a driving pattern most cars never see. Here is how to adjust for the one you actually have.
What the book assumes
Published intervals are calculated around mixed driving with the engine reaching full operating temperature for a sustained period. That is what burns off the fuel and water that condense into the sump, and it is why the same engine can be rated for 18,000 miles in one market and 10,000 in another.
Why short journeys are harder on oil
An engine that never warms through accumulates unburnt fuel and moisture in the oil. Both attack the additive package, and neither is measured by the odometer. A car doing four-mile school runs is working its oil harder at 6,000 miles than a motorway car is at 15,000.
Condition-based servicing is not magic
BMW, Mercedes and others calculate the interval from engine load, fuel used and temperature rather than mileage alone. It is a better estimate than a fixed number, but it is still an estimate based on assumed oil quality. If the car is used hard, servicing before the indicator asks is cheap insurance.
The cost comparison that matters
An oil and filter change is a small fraction of the cost of a timing chain, a turbo or a rebuilt engine. On any engine with a known chain or wet-belt weakness, halving the interval is the single cheapest reliability measure available.