Impossible to tell what type of rubber it has. I'd ask them. If they cant tell you I would buy a viton seal instead. As for your problem, hard to say. When engine hot, oil is thinner, and could well be leaking past seal. However, too much end float or rough face on flywheel will also cause problems. I posted my original post above because I was fed up with being sold cheap crappy seals. It doesn't matter if its german, a cheap seal is made of inferior rubber and in an aircooled engine you need decent high temperature rubber. Id wager that your seal is not viton rubber. In which case you will end up replacing again.
Just checked, NBR stands for Nitrile Butadiene Rubber
So not Viton. You pays your money you takes your chances.
"There are three sources of wisdom; illness, misfortune and enemies."