While doing my FAA ASEL in '94 my qualifying cross country flights from VNY to SBO involved reaching approx 10000' over the Sierra Nevada in a C152. I was young and much fitter then and didn't notice any altitude effects.Lots of leaning required but it is quite possible. When I returned to the UK I couldn't believe everybody was flying around at 2000ft and leaning seemed to be something a PPL shouldn't be encouraged to take an interest in. I know that controlled airspace is the cause of a lot of celing restrictions in the South and central UK but not everywhere. Nowadays I rountinely fly up to FL200 in my M20 (I prefer to be at FL100-120 where I do not need O2), but crossing frontal weather or Alps can sometimes demand otherwise and I use a portable oximeter intermittently while I have O2 on. I agree with maxdrypower that even fit youngish people can see O2 sats drop significantly at or slightly below 10000ft. If I start to lose the ability to do simple cockpit maths then the O2 goes on. My head starts to throb when the oximeter is the the low 80s (usually FL130 and above) but I routinely put the O2 on above FL120. As soon as I have taken a couple of deep breaths then the sats are back up to 99%.Anybody out there using cannulae instead of masks?SB