As I recall Rotax recommends using a high quality, major brand, 4-stroke motorcycle oil with gear additives and (SF) or (SG) API classification.
The gear additives are required to withstand the high stresses in the reduction gearbox. The (GL4) or (GL5) specification is recommended.
Full synthetic is not recommended if you are running 100LL more than 30% of the time.
I recommend consulting a qualified Rotax mechanic who probably has a better memory than I and is familiar with your particular aircraft.