"There is additives in the 91
"There is additives in the 91 and 93 Octane (in order to get it to that rating) that can be downright dangerous for an outboard.
There are a very few models on the market that can safely run 93 Octane so best to stay away from it unless your model specifically calls for it.
99% of outboards are engineered to run on unleaded 86 or 87 octane fuel (with upto 15% ethanol) and with 2 stroke technology, far more fuel goes right out the exhaust than is ever used by the motor.
Personally, I wouldn't even waste the extra few cents on the 89 Octane unless that was the lowest rating available (why argue with the guys who designed/built these motors).
As to running stablizer, yes, all the time (so says Merc, Honda and few of the other biggies in the game).
You should also ALWAYS run fresh fuel (no more than 2 weeks old) - So why run stablizer if you always run fresh gas? - it's not the gas in the tank that the stabilizer is there to protect, it's the residual gas in the motor after you shut her down that it will help stop separating/gumming up, getting full of "cooties".
When it comes to plugs, run NGK's - Merc and Yami ignitions were built with those plugs in mind - forget cross ref's, hotter plugs, platinum or irridium or split gaps etcs, go with what the service manual calls for.
Most 135 Merc's ran on BU8H surface gap plugs (Thunderbolt 4 ignition) or BPZ8HS-10's gapped at .040" (Merc's CDM ignition)"
