My lung function is in the 50's and I don't use oxygen, so this might not be super relevant to you, but here it is anyway. I grew up at an altitude of 2500-3000 feet. I now live at sea level, but returning to see my parents at their home is never a problem for me. I don't notice any difference. I spend time at a cabin around 7000 feet, too, and I do occasionally use oxygen there, and I need a break after climbing a flight of stairs.