AIUI, 91UL is just avgas without the lead, so the only risk is engine detonation, and tests done by GAMI suggests that in real life you need to work an engine awfully hard (with CHTs ~ 550F) for this to happen.
There is no risk to fuel system components.
The Swedish 96UL is more useful stuff and almost all Europe based GA pistons can run on it, but it isn't sold outside Sweden
AFAIK.
The various "100UL" propositions involve additives which any refinery can put in but they bring varying risks to fuel system components. And since most planes currently flying are orphans in terms of manufacturer support, this is a tricky area.