[Selkirk]

"'Hit don't make no diff'erns"

Those are the words of a diesel mechanic I work with. And when pressed, I have to admit, he made some sense.

Modern diesels run about 500,000 miles between rebuilds. That's about 10,000 hours.
So if you motor at 5 knots you'd have to drive your boat around the world twice in order to create enough wear to create a rebuild situation.

Just follow manufacturer recommendations and change about twice a year - even if you haven't used the engine. That's because oil is hygroscopic and absorbs water over time.
And be sure you are using an oil intended for diesels - it'll have a "C" designation (for compression ignition) instead of an "S" (For spark ignition) like gasoline engine oil.