Merc recommends their own blend 25-40 oil. The merc oil is not a true multi-grade oil, it is just a blend of 25 w and 40 w oils. A true multiweight oil would start out as (if 10-40 for example) 10 weight and they add viscosity improver so it will behave like a 40 weight when warm. The problem is as these multiweight oils break down over time and marine engines run hotter oil temps the viscosity improvers break down and you are left with a 10 weight oil.
I believe that Merc recommends straight 40W if not running the Merc 25-40 blend.
I'm running 20-50 because of the cold water temps up here. It takes too long for my oil to come up to proper operating temps.
The debate over synthetics vs petroleum oils will continue...