Sierra makes a 25w40 oil specifically for m ercruisers. You can get it in semi-synthetic or dino. I buy mine from Wholesale Marine off the internet. It is about half the price of Merc brand. I think the products are exactly alike. They also have a High Performance gear lube for Mercruisers.
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.
I think most HDEOs would be fine in any application that called for 20W-40 or 25W-40 as these oils are difficult to shear during any reasonable drain interval. And if the concern is fuel dilution, that larger cold-start number isn’t going to help you.
If your application is really tough on oil, I’d go with this Schaeffer synthetic blend in 15W-40:
Last I checked, it was a true 25% PAO blend … with the soluble moly add-pack that Schaeffer pioneered.
In addition to the rep on this site, Schaeffer now has an on-line store so ordering is easier than ever (although ordering in bulk from a dealer is usually the best way.
Want to go cheaper? I'd go with Chevron Delo 15W-40 or Pennzoil Long Life 15W-40.
I used the Amsoil 10w-40 Marine oil for the last couple years in our 5.7L Mercruiser. It's a little more expensive, but I keep it in there for two seasons. About $45 dollars for 5 quarts to my doorstep (not including the filter). I do change the filter and top off each season. We do alot of water sports, and the engine has always ran smooth. Used Royal Purple 15w-40 before that and liked it also.