Since you mentioned the FXT class, i assume you are talking about the USLMRA. If you will read the entire rule book, it very clearly states the rules on fuel under general mower requirements (which apply to all classes).
FUEL and FUEL DELIVERY
"The only acceptable fuel is pump gasoline. Fuel must be pumped fuel bought at Service stations, example SUNOCO racing fuel or VP Racing Fuel sold at Marathon Service stations. Additives, other than STABIL Fuel Stabilizer are prohibited. Any apparatus other than the carburetor which can be used to introduce any gaseous or liquid substance into the induction flow, whether connected or not, is prohibited, and will result in disqualification. Such devices include, but are not limited to, Nitrous Oxide, alcohol or water injection systems, turbo or superchargers, and/or the attendant hardware consistent with these systems. Any attempt to conceal or disguise such apparatus will be considered a flagrant violation and may result in expulsion from the USLMRA."