Mutt
I stand corrected, FL350, not FL 250. I am certain it was 250 back when I "wrenched" the Boeing Tri-motor.
Mansfield
You are quite correct on it being the most ignored rule. Bizjets routinely cruise for hours above 410 and rarely, maybe never, have one pilot on O2 for the entire time. We have a "cruise" mask that is supposed to be more comfortable for long-term use, it isn't much.
GF