Ask five people how often to change engine oil and you will get five confident answers, most of them inherited from a car that is thirty years old. Meanwhile the sticker in the corner of your windscreen says one thing, your owner's handbook says another, and the quick-change place says something shorter than both. It is worth understanding where those numbers come from, because two of the three have an interest in the answer.
What the oil is actually doing
Oil does four jobs simultaneously. It separates moving metal surfaces with a film thinner than a human hair. It carries heat away from the pistons and bearings. It holds combustion by-products and soot in suspension so they do not settle as sludge. And it helps seal the gap between piston ring and cylinder wall so combustion pressure pushes the piston rather than escaping past it.
The base oil does the first two of those for a very long time. What wears out is the additive package - detergents, dispersants, anti-wear compounds and viscosity modifiers - and those degrade on a schedule driven by heat, contamination and time. This is why oil that still looks acceptable on a dipstick can already be past useful life. Colour tells you almost nothing; a good detergent package turns oil dark precisely because it is doing its job of holding contamination in suspension.
Start with the handbook, not the sticker
Your manufacturer tested that engine, with that oil specification, and published an interval. That is the only number in this conversation produced by someone who both knows the engine and carries warranty liability for it. Start there. It will usually be expressed as a distance or a time period, whichever comes first - and the time limit matters just as much, because oil degrades in a car that mostly sits.
When shorter is genuinely justified
Manufacturers publish a second, shorter interval for what they call severe service. It is not a marketing device - it reflects genuinely harder duty. If most of your driving matches these conditions, the severe-service interval is the one that applies to you:
- Repeated short trips where the engine never fully reaches operating temperature, so condensation and fuel dilution never boil off
- Extended idling and stop-start traffic, where the oil spends a lot of time hot with very little airflow through the engine bay
- Sustained high ambient temperatures, which accelerate oxidation of the base oil
- Dusty conditions, which load the air filter and let more abrasive particulate reach the oil
- Towing or consistently carrying heavy loads
City driving in a hot, dusty climate ticks four of those five boxes. That is a legitimate reason to use the severe-service interval rather than the standard one - not because the oil is bad, but because your duty cycle is at the harder end of what the standard interval assumes.
The grade matters more than the interval
Here is the part that gets less attention than it deserves: using the wrong viscosity grade does more harm than being a couple of thousand kilometres late. Modern engines are built with tight clearances and rely on a thin oil reaching narrow galleries fast on a cold start. Variable valve timing systems in particular use oil pressure as a control medium and are specified around a particular viscosity.
Put a thicker oil in one of those engines - a common piece of well-meaning advice for older or noisier cars - and cold-start oil delivery slows, which is exactly when most engine wear occurs. Check the handbook or the oil cap for the specified grade and the manufacturer approval, and match both.
So what should you actually do?
- 01Find your manufacturer's interval - handbook, or the service schedule for your model and engine. Note both the distance and the time limit.
- 02Decide honestly whether your driving is severe service. Short urban trips in heat and dust usually are.
- 03Use the specified viscosity grade and manufacturer approval. This is not the place to improvise.
- 04Replace the filter at every change, without exception.
- 05Record the date and the odometer reading. Guessing is how intervals quietly stretch to eighteen months.
None of this is complicated, and it is by a wide margin the cheapest maintenance you will ever do. An oil change is a routine expense. The engine it protects is not.
