It really doesn't matter. 30 is probably recommended for cool temperatures, 40 for hot. A boat doesn't really see either. Cold starting is one reason to use 30, but cold usually means below zero. Hot is only a problem for air cooled engines, not an issue on a boat. See what the manual recommends for the transmission so you can use the same in each, unless you have one of those odd trannys that uses transmission fluid. If your engine uses a little oil then I would use 40, otherwise 30.
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Currently in Long Beach, CA