That's a bit of a contrast as most vehicles run better on 100% gas than oxygenated fuel (ethanol blends). I do get better mileage on 100% fuel (although not by much or really enough to justify the higher cost of 100%...peace of mind thing) but when using 10% blended ethanol (oxygenated) fuel, it still runs fine.
Alcohol can damage certain fuel system components and lines if the vehicle isn't designed to run blended. Any modern (at least the last 15 years) vehicle has been designed to run these blended fuels and may actually run "better" with the blended fuels. Some engines are designed to run the E85 gas (from 50% to 83% ethanol depending on region and seasons) but those vehicles will have a designation on them. Can't run E85 in the MoCo offerings....yet.
The M8 was designed to meet current and future EPA and European regulations to include running on blended, oxygenated fuels. My guess is the dealer may have meant to say it may run better on blended, oxygenated fuels because it was designed with that in mind. I'm betting it's dealer "fluff" trying to sound intelegent entelagent mo smarter.
They sell these things all over the world in places that has good to poor fuel quality so run what you want....just no leaded gas if you can still find it....that would be a no-no running a catalyst....