Some FBOs have a policy that prohibits fuel that has been removed from
an aircraft from going into any other aircraft. They'll use the
"used" fuel for AGE (ground equipment) but won't pump it back into an
airplane for liability reasons (could have become contaminated in the
airplane they took it from, perhaps?). I don't know of anything that
prohibits a private owner from defueling his part 91 aircraft and
refueling it with the properly stored "used" fuel. Common sense
requires the fuel to be kept free of contamination and proper safety
measures utilized in the refueling process.