Silly argument. Oil is cheap. Engines are not. To err on the side of excess
does no harm and may do some good.

Besides, when you change the oil in spring that you put in in the fall you
must first run the enine and bring it up to operating temperature to reduce
the viscosity of the oil so you can get it out. This serves several purposes.
The engine doesn't just sit there over the winter. It aspirates and water
condenses inside it. Because you have clean oil in there that water does not
form bearing-eating acid. The additives in the clean oil neutralize the acids.
Then running the engine with the fall oil flushes all operating parts. The
spring oil works fine over the summer because the engine is often at operating
temperature and condensation is little problem.

And yes, I have indeed chnaged the oil and then immnediatley chnaged it again.
Can't quite remember why but it certainly did no harm at small expense.

As for my car, it's a diesel too. It works very, very hard and I let my
mechanic look after it.

To repeat: Oil is cheap. Engines are not.

As for grade, Castrol Heavy Duty SAE 30. No multi-viscosity. That's for the
car. Why? Car engine runs from 800 RPM to 4,000 RPM and back over and over.
Boat engine, except when idling, runs a constant 2,100 RPM.