Since it is an experimental aircraft, you are running an experiment. So you decide what fuel to put in it just like you decide what avionics to put in it and what door latches to use. After flight testing it for long enough, if you’re still alive, the FAA says you can take passengers!
Putting a different fuel in would probably be a major change and require a trip to the FSDO and additional flight testing and a logbook entry at the end, just like when the plane was first built.
All that said, I know the EAA is working on the issue. So it’s possible they’ll get a special route approved somehow that avoids every experimental aircraft owner in the country having to repeat their flight testing…and bog down every FSDO with the paperwork required.