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"Also whether mixing 10W30 with something like 10W40 or other grades is ok?"
Totally. What you'll wind up with is two quarts of "10w35" so to speak. Maybe not precisely but close to it. You'll have some of the 30-modifiers and some of the 40-modifiers, and the results do average out.
WRT the actual addtive mix in any oil grade, your best bet would be to call the engine maker (or in this, Moyer Marine) and then call the company that is making the oil. All the name brands have tech support departments who can and will look up what additive package is best for your engine, and tell you which of THEIR products is best compounded for it.
I think if you called Mobil, Shell, Amzoil (ahem), a couple of top names you'd find very similar suggestions from all of them. There are things that are not obvious when you reaed the can, or the web pages. Zinc may be used as a lubricant--but the Mobil synthetics have been used more expensive Molybdinum powders for 20+ years now instead, without mentioning it.
API and SAE also answer questions about this stuff, they LOVE it when a customer calls in and tries to learn what oil really is going to work best.