Bushy, your lament re altitude limits reminds me of the fact that back in PNG we regularly operated well above 10,000' in C206s and BN2s without supplementary oxygen.
Any enquiries from anyone unfamiliar with PNG ops as to whether either of the abovementioned A/C were equipped with oxygen would often be replied to by laughter.
I once spent just over two and a half hours in a 'A' model C402 at what I considered a ridiculous altitude without supplementary oxygen and had my reservations confirmed by suffering an altitude headache upon landing at Mt Hagen.
I discovered a good cure for it, but that's another story!
