I went right to the Rotax expert for this question.
You are supposed to check the magnetic oil plug every oil change per Rotax ASB-912-051.
You don't "PURGE" and engine at the oil change, you "VENT" it. The oil purge is only done when you dump the oil out of the oil hoses, oil cooler or like when the Rotax 5 year hose change is performed. This requires the inlet hose to the tank to come off, that port on the tank to be blocked and air pressure applied to the tank and then the prop rotated to "RE-FILL" the entire oil system, remove air and make sure the tappets have oil..
When you do an oil change all you do is vent the system. It just helps prefill the oil filter and remove some air back to the tank which is the only engine vent to the atmosphere.
The manual says to clean the oil tank every 200 hrs. If you use all 100LL this is important. Not so much so if all you use is 91 oct. Lead accumulates at the bottom of the tank and around the bottom of the screen. There is no good reason not to clean a tank and that comes right from Rotax. When I questioned this this is the video they sent me to watch. They told me using the wrong oil will cause this mess and running too long an extended oil change interval.
If all you do is remove the top two oil fittings on top of the tank and do not let them hang and lose any oil then all you have to do is replace it, fill it and do a standard oil change "VENTING". The reason told to me is that when you open the bottom oil tank drain all the oil up to and including those two fittings drain out anyway and none comes from the hoses. I just use a plug in the hose ends while I clean the tank and I haven't lost more than a drop of oil.
The gearbox does not get rebuilt at the 600 hr. mark from using 100LL more than 30% of the time. They told me that if you use 100LL a lot then usually by 800 hours the clutch is pretty clogged and even though we can't tell the difference it probably would function properly from all the lead. The overload clutch comes out and sent to a service center and it gets disassembled and cleaned due to lead fouling. The other parts are washed off, but should be fine. I was told when they re-assemble it they usually replace the bellville washers and reshim to its proper spec.