The most important word is “or”
Open almost any service booklet and the intervals are written the same way: every 12 months or 15,000 km, every two years or 30,000 km. It reads like one rule. It is two, and the service is due at whichever one you reach first.
Written down, that sounds obvious. In practice most of us track only one number — the one in front of us. The odometer is always on the dashboard. The date of the last oil change is in a drawer somewhere, if it is anywhere.
Why time matters even if you barely drive
Some parts of a car age whether it moves or not.
- Engine oil. Oil breaks down and collects moisture and fuel over time. Short trips where the engine never fully warms up make it worse — and a car that does little mileage is often doing exactly those trips.
- Brake fluid. Most brake fluid absorbs water from the air, which lowers its boiling point. That is why many manufacturers give brake fluid a time interval and no distance at all.
- Rubber and coolant. Hoses, belts, wiper blades and tyres harden with age, and coolant additives wear out. Mileage has little to do with it.
For a car that sits most of the week, the calendar is usually the deadline that arrives first.
Why distance matters even if the car is new
The opposite driver — long commute, lots of motorway — reaches the distance limit long before the year is up. Brake pads, tyres, filters and oil all wear with use. For this driver, a once-a-year service habit means running well past the interval every single year.
The low-mileage trap, in numbers
Take an interval of 12 months or 15,000 km and two drivers:
| Driver | Distance limit reached | Time limit reached | Due first |
|---|---|---|---|
| 6,000 km a year | after 2.5 years | after 12 months | time |
| 40,000 km a year | after ~4.5 months | after 12 months | distance |
If the first driver waits for the kilometres, the oil stays in the engine a year and a half longer than intended. If the second waits for the annual service, they cover almost three intervals’ worth of distance on one change of oil.
These numbers are only an illustration. Your car’s real intervals may be quite different — check the owner’s manual or ask a qualified mechanic.
How to keep track of both
You don’t need special tools. Each time something is serviced, write down three things:
- The date.
- The odometer reading.
- The interval — both parts, as your manual gives it.
Then work out two due points: the date plus the time interval, and the reading plus the distance interval. The earlier one is your real due date. To turn a distance into a date you need to know how far you drive in a typical month — your fill-ups will tell you, if you log them properly.
A spreadsheet with one row per service item works fine. Its weak spot is that someone has to open it. A reminder that arrives on its own beats a perfect spreadsheet nobody looks at.
Check your own schedule
Intervals differ between makes, engines and model years, so use the numbers in your owner’s manual or service booklet rather than any general rule — including the ones in this article. Many manuals also list a separate “severe” schedule with shorter intervals for frequent short trips, towing, dusty roads or extreme temperatures. If that describes how you drive, it is the schedule that applies to you.
The short version
- An “X months or Y km” interval is due at whichever comes first.
- Low-mileage cars usually hit the time limit; high-mileage cars hit the distance limit.
- Record the date and the odometer at every service, and check both due points.
- Your owner’s manual beats any general rule.
