Dan J,
I thought the same about mixing but Mercury has said with the two oils (Premium Plus and Opti synthetic) it's perfectly OK! This is due to the fact most bass boats carry 2+ gallons in the tank and another quart or so in engine reserve tank under the cowling and it is totally imposible to change over oils without some mixing going on (unless you plan to run "your" motor without oil to completely clear it out, NOT!) So, if you start out with either and want to change over some day it's not gonna be a problem. Once again, just info directly from Merc and anyone that is not covered under warranty by them would have to decide that if they felt it was a risk they shouldn't change from one to the other.
Yeah, we still have Chevron stations all over northern California and yes, we HAVE to buy these Optimax motors, someday you other Americans will catch up to our regulations, you always DO! heheheheheh
Tom