It is important to make sure that when filling up that the fuel pump or tank next to the pump has a 91 octane sticker on it. Mogas does not nesseccarly mean 91 octane. A lot of times it mean 87 octane which is a no no for a Rotax 912. On the web links like Airnav they don't say which octane is available next to the word Mogas.
As John says, you have to watch the octane. I used to carry octane booster with me when I travelled. However, I just gave up and I use 100 LL with Decalin instead. Easier, no worries, and approved by Rotax. At home, I burn Shell 93 E10, but on the road, I just use 100 LL and change the oil more often.
Several times I've made specific fuel stops just to get mogas, only to find out that the pump has 87 octane. Pretty much forget that now and use 100LL + decalin when on long cross country trips.
I am curious about Decalin. Where do you get it, what's the cost and how do you measure the right quantity to add to your fuel tank after filling with Avgas?