Confused about which oil weight to use
The shop manual for my Yanmar 2QM15 specifies either 10 or 20 straight weight oil, depending upon outside temps. I realize that when the manual was written, modern multi-weight oil didn't exist, but my understanding is that because the engine was designed to operate with 10 or 20 weight oil when at operating temperatures, I should use a multi-weight oil with a similar operating temperature weight (something like 5W20, for example).
Problem is that the readily available diesel oil doesn't seem to be available in this low viscosity. For example, Rotella T, which many recommend, comes in 15W40 and 10W30, both of which would seem to be too thick.
To add to my confusion, since this is a raw water cooled engine, it operates much cooler than a fresh water cooled engine, making the use of too thick of an oil even more of a problem.
What am I missing in this equation?
1981 Spirit 28 "Bonnie Lass"