I am going to post some excerpts from rotax:
The 912 may be operated using leaded fuel, but this is not recommended as lead sludge tends to accumulate in the oil tank and reduction gearbox. Also, avgas is incompatible with the recommended synthetic oil which cannot hold lead in suspension; consequently, the use of leaded fuel mandates additional maintenance.
According to the Rotax 912 maintenance manual, Oil changes should be done at 100hrs and plugs changed at 200hrs when using unleaded fuel.
If you use avgas, then it is recommended that you change the oil at 50hrs and plugs at 100hrs.