effects of hypoxia can begin at 8000 ft
I don't see anybody disagreeing with the above.
Just avoid flying commercially, if you see obvious symptoms at 8k. And perhaps see a doctor to make sure there is nothing seriously wrong.
I use o2 at 10k on long flights, say 2-6 hours. Keeps me fresh and on the ball.
Effects vary. I once flew with an instructor whose vision was too blurred at 12.5k to read the instruments. Within seconds of getting o2 he could see fine. The FAA recommends o2 at night above 5k - that's a specific effect of reduced o2 on night vision.
But I maintain that anybody feeling noticeably ill at 8k should get looked at. And avoid commercial airlines. It might save their life.
Cant believe I am studying to be one of them
If you are studying for an ATPL and you get noticeable hypoxia at 8k, you are wasting your time, because that is what your airliner will be at.