I have a 48 hp Evinrude that I bought new 32 years ago. In all that time I've run only OMC conventional oil and either Stabil or OMC 2+4 conditioner (which used to have an MSDS identical to Stabil, even made by Gold Eagle). This spring it went in the shop for the first time for carburetor rebuild to fix a 32-year accumulation of fine crud. In the process of troubleshooting what caused it to drop a cylinder the compression was checked; good in both cylinders. I see no reason to change oil.
Not just with outboards, but with other engines in general, I've always figured that the engineers who designed something know best, and that most products marketed by the manufacturer of an engine are well-suited to that brand of engine. I've had similar results as above using Stihl oil in my 41-year old Stih saw and an Echo trimmer that's about 30 years old.
Back in the 70s I had a friend who generally ignored manufacturer recommendations, as well as the outboard mechanic at work, who was OMC certified. He put some hot rod aftermarket prop on his almost-new 135 Evinrude and promptly blew it up. It could have also been because he ignored break-in recommendations and ran top end all day first day in the water.
