Marc,
Your experiment appears to confirm our thinking that we should be
oxygen when flying above 10K. In our last several long cross-country
trips, we flew at 9K and 10K approximately 3-4hrs stretches and did not
feel tired or any differences from flying at lower altitudes. Others
may need oxygen at lower elevation. I knew few heavy smokers with
limited lung capacity having to drag oxygen bottles along all day. Of
course the oximeter is a very useful device but oxygen supply is the
key to our health and safety. I'm not at all against the use of
oximeter but would certainly get O2 bottles before buying an oximeter.
Come to think of it, yawning is a pretty good oximeter too ;-)

Hai Longworth