Re: Oil viscosity
If a 2 cycle OB, a BIA certified TCW3 rating (currently popular) is all you need. Current Mixing for most applications is 50:1 or a pint to 6 gallons of gas. They never tell you what the viscosity is and some put additives in it to improve performance that can affect the "apparent" viscosity if it doesn't actually affect it. I know Quicksilver oils are like that.
On 4 cycle Ford/Chevy marine adaptations, don't know. I know Merc markets an oil specifically for Marine engine applications. When I was young a 500 mile break in was a big thing, but that was when the only oils you could get were straight weight. In the last 20 years or so haven't heard anything about changing oil types for break in. Have read remarks like to take it easy for the first 500 miles and don't stress the engine.
On a boat that would translate to me as motor out past the no wake buoys, firewall the throttle to get on plane as soon as possible. Cut throttle back to a medium rpm area and spend most of your time there with intermittent WOT bursts short at first then increasing as the engine gets some time on it. 10W-30 seems to be a popular oil, and recommendations that I have read is to NOT use Syn or Syn Blend oil on the initial fill up. You want the engine to "wear in".
My 2c,
Mark