Gotta ask, what altitudes were you at? I've only been up to 10k in the 60. I know a few guys who did high altitude testing in it, but that's it... Super curious now.
I've only been above 10K in the -60 twice. Once was a very real operational need to 11.5K, no questions asked, the other was discussed as a crew and we agreed it was worth any risk (which was nil). But neither of those times did any of us experience any notable hypoxia (obviously just due to physiology, we were slightly hypoxic just due to altitude).
My personal two instances of full-blown hypoxia were in the T-34 at ~15K, once as a student and once as an IUT. Both times we were doing BIs in the MOA. I just don't have a mask-friendly face unless I go all the way to 3-clicks, so while looking down at the gouge on my kneeboard card repeatedly, my seal was slightly broken and I wasn't cycling the regulator. Fortunately, I recognized my one symptom that I get (general mental fogginess) after what I would guess was 4-5 minutes of hypoxia setting in, and I looked at the blinker and saw it wasn't moving. After readjusting the mask and going to 100%, the problem was solved and I didn't even bring it up to the IP.
The second time it happened, it triggered the memory from the first time, and I knew exactly what was going on and fixed the issue. What I did find interesting is that I still was unaware of the issue for probably the same amount of time as an IUT as when I was a SNA, despite having a similar experience (albeit 4-ish years earlier).
