I spoke to mercury today, they said that since I’m in a cooler environment I need to stick with 10w-30 but if I were to move to a hot climate the best oil would be 25w-40 synthetic blend and if I switched I need to stick with that oil. I asked why the sea pro used 25w-40 oil and he said it was because it’s meant for commercial duty. I asked what the difference in the blocks were and he stated the sea pro is a de-tuned 150 to run reduced rpms. Seems a little contradictory but I guess if a motor is pushing around heavy loads all day long you don’t worry so much about cold starts(?)
Seems contradictory to me as well. You stated that you never go out unless it's 50+. This would make me personally lean towards the 25w40. Either way, I do not think a 10w30 is going to cause you any misfortune. Thank you for posting Merc's response!