They say ethanol will be corrosive to aluminum and can cause some rubbers to swell and/or deteriorate. Most O-rings in a FFV are made of Viton rather than neoprene for this reason.

But, I've known lots of people that are blending their own ethanol/gasoline ratios to achieve a certain octane, and jetting their carbs accordingly, and not one has shown any problems of any kind. There's a farmer near me that's running E85 in his old John Deere "B" (the old "Johnny Popper" 2-cylinder), and he's had great success with it. I helped him tune for it, since with ethanol, you can't go by plug color, or whether or not it pings, or anything like that...You just tune for maximum power, and easy starts. Ethanol doesn't ping under load like gasoline, and doesn't discolor the spark plugs much at all. Only way to really know if you're running lean is by the exhaust and/or combustion chamber temperature. At first this tractor was way lean, and the exhaust pipe (nice shiny chrome) turned blue! Turned the main jet a half turn out, and it started to run smoother under load, and didn't seem to be quite as hot out the pipe. Not 100% sure if it's dead on or not, but it runs great.

The place you're getting the E25 must be similar to the stations I fill up at. In the ground, they have one tank of straight ethanol, and one tank of straight gasoline, and the pump changes the blend ratio for E10, E20, and E85.