E-10 can safely be used. But you have to use it wisely. Ethanol is only part of the problem. The two factors that we see with E-10 are:
1- that the alcohol is a solvent, which can dissolve/clean/dry everything it touches, and
2- that it is hydroscopic (attracts water).
That means that any rubber or plastic bits in the fuel system must be tolerant of alcohol. Fortunately for us, modern outboards' fuel system's components can tolerate E-10 without becoming dry and brittle, or melting into goo. However, it also means that you cannot reasonably store atmospherically-vented E-10 fuel for more than about 30 days without some water contamination. In severe cases, the fuel can suffer "phase separation", where the alcohol/water drops out of the fuel, leaving low-octane gas on top (because the alcohol is used to boost the octane of the gas). If some of that alcohol/water makes it into the carb bowl, it will sit there (being heavier than gas), and corrode the metals of the fuel system.
That said, all of the "pump gas" we get today (with or without alcohol in it) is lower quality than we used to get 30 or 40 years ago. It's not as stable (spoils faster), and it leaves more varnishing residues as it evaporates. This is also true of some other fuels, including some K-1 kerosenes. For example, I have a glass "hurricane lamp" that has a lot of orange coating in the glass reservoir that has built up as the kerosene has slowly evaporated. That same artery-clogging residue builds up (in microscopic layers) in our carb passages, if the fuel is allowed to sit in them and evaporate. In short, today's fuel is lower quality than what we would like for our lightly- or infrequently-used outboards.
Then add the EPA emissions requirements. They end up forcing the outboard manufacturers to run the engines a lot leaner that they would like. Less gas = fewer emissions. In the case of carbureted motors, this is accomplished by designing carburetors with longer, narrower passages. This causes a better, more thorough emulsifying or mixing of the fuel with air, which makes it possible for the motors to run at these very lean mixtures.
So, we have a double whammy: Poor gas that leaves more residues, and carbs that are more succeptible to flow restrictions from those residues. In both cases, you can thank your elected "leaders" for our woes.
The solution is pretty simple. Use only fresh gas, and never allow it to sit in the carb when you aren't running the motor.
To prove that recommendation, I bought a new carbed OB, ran it on ordinary pump gas for a summer season, treated the fuel with storage stabilizer for the winter, fogged the motor, and shut it down, leaving the motor on the boat, and the carb full. It was running perfectly, with excellent idle, great acceleration, and no stumble whatsoever when I shut down for the winter. In the spring, it could only barely start and run, and definitely would not idle properly. The carb had to be cleaned in order to restore proper running. Subsequently, I have religiously run the carb dry at the end of each day, and have not had to touch the carb in the past 3 years. My customers' experiences have verified this recommendation as well.
Of course, there are occasions when, even with the best of intentions, the "ideal" procedure isn't followed. Then we have to clean up the mess.
In mild cases of slight idle issues, or slight acceleration stumble, it may (or may not) be possible to have success by running a super-treatment (say 10X the recommended dosage) of a fuel system solvent (such as chevron techron) in the gas for several hours of near-idling... or to disassemble the carb and blow out the passages with generic carb spray. But usually, by the time we notice the problem, it is much worse. In that case, a "real" carb cleaning is called for. That means a complete carb disassembly, a 4-hour soak in real carb dip (to dissolve the residues), and then a thorough blow-out of each and every passage with generic carb spray (to remove the dip and the dissolved residues). Then of course, a careful reassembly, paying close attention to proper settings, such as float height. In our shop, we have successfully used CRC Tyme, Berryman's, or Gunk branded carb dip products. The hard plastic parts of the carb are not damaged by the 4-hour bath.
HTH (hope this helps).
