On a related note, if you run out of oxygen, you can get yourself some extra altitude margin in a pinch by pressure breathing. I rather discovered this while swimming laps. I could do better with a controlled pressurized exhale. I figured the same idea might transfer to soaring. I did some reading that supported the premise and then did some experimenting at altitude. It definitely works (legalities aside). Having 45 years of western soaring under my belt, I have a better than average sense for my own personal oxygen requirements. My tentative conclusion is that, for me, pressure breathing is worth around three thousand feet. Certainly I'm not recommending pressure breathing in lieu of supplemental oxygen, but it might be a useful thing to know about just in case.