The Rotax 912 is a popular combustion engine in smaller general aviation aircraft in Europe. Due to a number of factors (including fuel cost), the Rotax-powered aircraft in our flight school are most often run on MOGAS (Super 95). Since smaller airfields don't always have MOGAS available, they are frequently flown on mixtures of AVGAS and MOGAS during longer flights.
Going through the technical documents of the engine, I never found a passage mentioning explicitly that these different fuels can actually be mixed. I was wondering why that is, considering the great length to which the manuals go to cover all potential combinations of operating fluids (lubricants, coolants, fuels).
The authoritative reference for the approved fuels of this engine is the Service Instruction Document
Rotax SI-912-016R14 (linked here is a slightly outdated revision).
It does mention:
The following fuels can be used.
and goes on to list MOGAS and AVGAS of different octane ratings. The only passage where mixing of fuels is mentioned explicitly cautions against a potentially low octane rating of the mixture:
Any mixture of unapproved fuels and/or additives that cause lower than the specified octane rating can cause engine damage like e.g. detonation.
Not having a background in combustion engines, I can only speculate that perhaps it is implicitly clear that any mixture of approved fuels would also be safe for the operation of an engine?