NO problem–
*It can be mixed in any percentage with 100LL, so if a pilot wants to top off a fuel tank that already has 100LL with G100UL, there will be no problem. “The fuel in the plane will still retain its spec and integrity and there will be no degradation in performance,” Roehl said.
( George Braly and Tim Roehl -owners of General Aviation Modifications Inc. (GAMI)