I use a pulse oxymeter on flights over about 8,000 feet. I regularly cruise at FL100 or FL120 in the Cirrus but putting the oxygen masks on is a huge pain so I monitor my O2 levels and avoid it if I can.

I did an experiment recently on one flight where I got all my PAX to test their blood o2. Scores varied between 78% and 96% with no obvious explanation for the variation (i.e. no smokers or people with colds). This makes me think that there’s no general o2 level that works for everyone and that we probably need to understand our own personal reaction to hypoxia and when it sets in. But how?

On another quick point, I think you probably need to consider supplemental oxygen above 5,000 feet or so at night because hypoxia can reduce night vision even at relatively low levels.