My 2006 Merc EFI 75~4 stroke clearly states in the owners manual not to run anything over 87 octane, given that ethanol free is not an option in that grade I had my marine mechanic install a fuel/water separator on the gas line just ahead of the engine, many years ago. That along with always using stabilizer I've never had an issue and last year I was running on 3 year old gas without a hiccup for a better part of the summer. I believe it was money well spent.