My manual specifically says in relation to fuel the following:
Preferred Fuel: Any regular unleaded, regular leaded, or premium unleaded gasoline having the recommended octane rating, and not extended with alcohol.
Acceptable Fuel: Any of the above gasolines with up to the following percentage alcohol by volume: • 10% ETHANOL or • 5% METHANOL with 5% cosolvents.
My motor is a 1994 Johnson. Not sure how long alcohol extended fuels have been around, but it looks like it's something that outboard manufacturers at least had in mind and accounted for when building their engines at least back as far as 1994.
BUT, my manual also says the following about alcohol extended fuels:
OMC products have been designed to operate using the Preferred Fuel or Acceptable Fuel; however, be aware of the following: • The boat's fuel system may be different regarding use of alcohol fuels. Refer to boat owner's manual. • Alcohol attracts and holds moisture which may cause corrosion of metallic parts of the fuel system. • Alcohol extended fuels can cause engine performance problems. • All parts of the fuel system should be inspected frequently and replaced if signs of deterioration or fuel leakage are found. Inspect at least annually.
I don't have the luxury of any alcohol free gas available in my area. So I use premium gas with the appropriate amount of Stabil in my tank and haven't had any issues thus far.
As far as higher octane fuels, my manual goes on to say this:
The use of premium grade fuels is specifically encouraged. Most national brand premium fuels ~ contain detergent and dispersant ingredients advertised to reduce intake valve and fuel injector deposits. These ingredients also remove and prevent carbon buildup on pistons and rings. These ingredients can extend engine life while maintaining a high level of performance.
Now all of this is obviously coming from the manual for MY specific motor and model year. I'd encourage everyone to read up on what THEIR engines manual recommends and make the best decision based on the manual regarding fuel grade and alcohol extended gas.