Don't forget, Rotax don't make their O rings themeselves, they choose a suitable material from those available from O ring manufacturers and then order what they need. They've chosen FPM 75 which is Viton (Viton is a trade name, there are others that make the same chemical compound and give it their trade name). I worked on machinery for oil refineries and we used Viton a lot because it was very very good for petrochemical applications. The only risk you run is if you buy it from a very cheap supplier you may end up with a pirate material (Rotax run the same risks unless they actually test batches of O rings they receive). I came across Metrics recommended in another forum for Europa aircraft so I think they're a pretty safe bet. I think you'll find that the Metrics O rings are a fraction harder (shore 80 instead of 75) but that should change nothing in your case.
In the worse case what do you risk? You might get a small leak and a dirty engine.
If you google "O ring material compatibility" you'll get lots of charts where you can compare Viton to other material like Neoprene. I think you'll find Viton is a better material for this application but Neoprene will probablly do the job and may be cheaper.
If you google "O ring 105 x 2.5-N FPM 75 you'll also find the same Rotax O ring for Rotax Go karts much cheaper than for the 912/914.
As I said above the important thing is finding a trustworthy supplier.
Mike G
