That would mean that with your engine in a trawler you'd need to change the oil every 2 days on passage.

Ok, so you wouldn't put that engine in a trawler but every 50 hours? We motorsailed up the coast for 4 days (non stop). If I changed the oil at 50 hrs I'd have to stop and drift while I changed the oil.

Our old Perkins is a slow rotation engine (cruising rpm is 1,600). The manual says every 250 hrs. I start to think about it at 100 and make sure it's changed by 150. Of course, we have a motorsailor and as you would guess use the motor quite a bit. If the wind is under 10 the engine gets started. No reason to go 3.5 knots for 200 miles when I can go 7.5.

I would only guess but think if you're using the engine for 50 hours straight without turning it off (or shutting it off only once or twice) the 50 hour rule doesn't apply. The manufacturer might have put 50 hours in because most people with that engine don't run it continuously but in short spurts and engines don't like that very much.

If I was crusing with that engine I'd change it at 100 hours and do what 'John' does. If it shortens the life of the engine, maintain it in all other respects. Unless you keep your boat for life you'd probably sell the boat before the engine wears out. We have more than 7,000 hours on TD and the Perkins runs great.