yeah that was my first thought too, but the club has a strict rule only Unleaded. They mentioned why, something to do with the carbon deposits on the cylinder walls and that they've had problems with that in the past.
Sounds strange to me..
We've had 912's here that have logged over 1,000 hours on a combination of AVGAS and MOGAS without problems..
My former instructor does over 300 hours a year on his 912 with mostly AVGAS...