Short answer
Use your engine's manual and act on whichever limit arrives first — running hours or calendar time. A low-hour engine still needs its annual service, because oil, coolant, impellers, belts and anodes degrade on a calendar regardless of use. Record hours and dates so the next interval is never a guess.
You will need: Engine manual, hour meter reading, logbook or spreadsheet
There is no safe universal interval, and anyone who gives you one without asking which engine you have is guessing.
Build the schedule from two clocks
Hours drive oil, filters, valve clearances, injectors and heat exchanger work. Calendar time drives coolant, impellers, belts, hoses, anodes and drive lubricant, because those degrade whether the engine runs or not.
Whichever limit arrives first, that is the service. A yacht auxiliary doing thirty hours a year is on the calendar clock almost entirely — see how often a marine diesel should be serviced.
Setting it up once
- List every scheduled item from your manual with both its hour and calendar interval.
- Record today's hour-meter reading and date against each one.
- Add the next-due figure for both clocks.
- After every service, update hours, date, part numbers and what you found.
That last column is the one that pays. A record showing which filter, coolant or wiring change preceded a fault turns diagnosis into reading rather than guessing.
The Australian modifier
Tropical and subtropical conditions shorten the practical interval for anything in the raw-water and exhaust path. A schedule written for a temperate climate is too long in Darwin or Cairns — see service by region.