The O2 rule for the pilot alone in the cockpit hasn't been SOP for around 15 years anywhere I've flown
Fine - but, in the situation, I would choose to don and use my mask until the other guy/gal gets back and, if I'm in the left seat, he/she would be directed to do likewise. Guess I'm just a dinosaur ?
I wonder if the relaxation in the operational rules has anything to do with civil folk not routinely doing chamber runs so that they can get the experience of what a severe hypoxic incident is like .. and observe the variability amongst people in their hypoxic responses ? Like a number of things, one really needs to walk the walk to get an appreciation of the story .. sometimes talking the talk just doesn't cut the mustard.
I can't comment on the current operational rules, specifically, as I'm a tad out of touch with them. However, using the mask didn't cause us a problem in times past and, to me, it is a QED risk exercise matter.
The fact is that hypoxia has an insidious fatal track record which, in my view, is of sufficient frequency to warrant a conservative approach to the problem.