Merc lists several oils that can be used, and in the order of preference.
Here is a link to the Mercruiser site. Read the whole oil recomendation section of the link, not just the big words.
http://northamerica.mercurymarine.com/serviceandwarranty/mercruiserfaqs/fueloil.php
This thread is like every other oil thread. Every DIYer seems to know more about what oil to use in their marine engine than the manufacturer. However, that same DIYer can't explain the basic principals of how an engine works, yet he knows all about oil.
Kind of funny really. If some DIYer uses XYZ oil, and the engine didn't blow up the first time out, then it must be the best oil in the world, and he will recommend it to everyone saying it's the best thing since sliced bread. Then he looses his engine the next year with a cracked block because he didn't have the sense to winterize properly.