For diesel engines, it's tough to beat Delo 100 or Delo 400.
I use a single weight oil (30), because it's almost never colder than
50 degrees in the engine room when I start my boat. I also want as much
"oil" in my oil as possible, not all the parrafins and other junk they
add to runny, low viscosity oil to make "multigrade".

If you get lazy about following the recommended oil change intervals,
it's a bad idea to use a multigrade. All the viscosity improvers break
down over a period of time, and you are then left with only the lowest
number on a multi-grade oil. Pull the dip stick on an engine that has
gone way too long without an oil change at the black goo runs off like
dirty water.

But always follow the mfgr recommendation. If the builder says to use a
multi, use a multi- but be sure you also follow the recommended change
intervals or change even more frequently.