Like I said in my post above... my old Marlin 21' Daycruiser has around 1200 hours with the OMC King Cobra and I started using Mobil 1 in it the second year I had it. It still has near stock compression on it. And the engine just purrs...That is enough to sell me on it. And my neighbor that bought it loves it and knows it was well cared for.
With synthetic oil, the oil coming out looks better than any petroleum oil, does not have that burnt smell and still feels slick.
There is a noticable difference in the power produced by the engine, albeit not huge.
I find it interesting that after years of insisting on single weight oil, Mercruiser came up with a multi-viscosity oil in a weight that no one else makes. Personally, I think that was designed to create a captive audience...but then I am cynical.