50 weight oils (including XW-50s) are over-used and are usually only required when you encounter the highest oil temperatures.
A heavier-than-necessary oil robs power and fuel economy and will not necessarily protect any better.
Recommendations from Red Line Oil regarding their 15W-50:
"Not recommended for street use in production engines that see sustained oil temperatures below 225°F (those engines should use Red Line 10W-30 or 10W-40)."
I'd use the same advice for other brands of oil.
lars, have an oil temp guage on your boat? Water temp? Do you know the highest temps you have seen to date?
Have a regular cooling water temp gauge, and it has never been above 175F.
Note: The Volvo Penta Gas engine oil is 15w50 and API SG.
I have access to diesel oil with the following grade:
API CI-4/SJ, ACEA E7/E5/E3/B3, MB 228.3, Volvo VDS 3, MAN 3275, Allison C-4.
"Have a regular cooling water temp gauge, and it has never been above 175F ... The Volvo Penta Gas engine oil is 15w50 and API SG."
I'd expect the oil temps to be a higher than the water temps as you will not get perfect heat transfer and the water starts out much colder when it comes into contact with the engine block.
But it seems to me that a 50 weight oil is a bit thick for this application if you factor in Red Line's advice. Does the manual offer you a temperature range and different oil weights for different temps or is it one size fits all? They probably spec the one weight for all uses. I really hate that. There's a better answer if they put some thought into this.
The 50 weight seems too thick for cold-water (not tropical) applications ... and they may be even assuming some shearing from thermal, physical and/or fuel dilution.
But if you change the oil at least as often as recommended, I suspect you'll be fine with a 15W-40 HDEO.
I suspect the diesel oil you can get with those specs should be fine for normal use. For heavy-duty or high-speed use, shorten the change interval by at least a third.
I suppose it may get some fuel dilution even though it is an injected engine. Shear, I don't know the rate. Its cruise rpm is 3500 and max rpm is 5000 which is very rare for me.
OCI is 50hrs in manual and that's what we do per season.
Can I use the oil pressure reading to determine if it diluted or sheared too much before the 50hrs?