I have done plenty of research on this. It is how i found bitog. The current Mercruiser syn 20w40 is made for boats with catalytic converters. It is light on additives to protect the cats. I would not recommend it on your application. If you are inclined to use Merc, use their 25w40 blend. I think there are better choices for you though. Any really stout hdeo would work well, straight 40 or 30 if you like, or a 15w40. Delo seems to have the best add pack if you look at voa's. I looked at brad Penn racing 15w40 syn blend. It has the most Z and P, and the highest hths by far, but is hard to get. I can buy it at the refinery directly for a really low price.
I purchased delo 15w40 last night online from AA for $9 gallon, so I will likely use that for the mid season change, but I think my first choice would be the brad Penn, price being equal.
BTW, I have a 29 footer with a 350 Mag Merc.