I've been using Mobile 1 in my 5.7 Mercruiser since I bought it with 211
hours on it. I use the green top 10-30W variety which is supposed to be
for older motors. I believe part of the reason Merc doesn't want you to
use it is because they don't sell it. Their pitch is similar to other
fluids for their equipment.

I change every 100 hours (three times this year, I'm happy to say!) and
use a WIX/Napa gold filter. Ask me in 3000 hours whether I was glad to
have used synthetic....

Jeff