GAMI and the FAA have both stated that G100UL is safe to mix with 100LL in any proportion. The fuel distributors need to do nothing to clean out their systems. Simply fill up with G100UL and by the second or third load, the amount of lead in the tank/truck will be negligible.
The FBO’s / distributor’s lawyers can simply point to the FAA STC and say, “we’re not liable for anything that happens with this fuel, it’s FAA approved.” What’s wrong with that?