Facts about ethanol (specifically E10):
- Has only a 3 month shelf-life. You should be turning over a full take at least once every 3 months.
- Is much more susceptible to to water and moisture from high humidity.
- Lowers MPG and is less efficient.
So given the above items, you can't exactly claim it's a slippery slope to say that E10 could cause problems in a system not specifically designed for it. Also consider that e10 exists because it is a happy medium between content and usability in non-designed-for-ethanol-applications. If ethanol was completly safe for existing engines, we would have gone up to E20 to start instead of E10. It's also been proven that some gas stations have more than the 10% allowed.
General signs of high alcohol content in fuel when run through an engine not designed for it:
- Stalling.
- Hesitation during WOT.
- Smoke released from exhaust.
- Clogged fuel filters and carburetors.
- Damage to fuel and VRO pumps and pistons.
- Dissolving of engine parts (especially rubber and plastic).
- Drying-out of parts (hoses)
- O2 sensor damage
- etc