You might get some helpful info from this presentation. It gives some details on the three major types of O2 delivery systems. http://aviation.derosaweb.net/presentations/#oxygen

Personally, being a flat lander, when I go out West I begin breathing O2 as soon as I am strapped in. This is way beyond what the FAA requires but it sure beats getting headaches at altitude...or worse.

I to use a Mountain High system (O2D1). Sometimes they have the older refurbished models for sale cheap - see https://www.mhoxygen.com/closeout-refurbished/.

- John OHM Ω