Volvo says we shoud change the oil in our 2003T engine every 50 hours.

That makes sense to me if we are using the engine mostly in and out of marinas or anchorages, with the occasional spell of several hours motoring .

It is well known that short runs cause build up of condensation, acids etc. while these boil off during extended running under load

We are currently moving North in Norway, with almost continual zero to light winds on the nose, in narrow channels, so are motoring far too much.

How long is it reasonable to leave between oil changes in such circumstances?

How long for filter changes?

(we normally motor about 2300 rpm, which is aboiut 75% of max)