Hi MrsFord - First, let me commend you for using the treadmill, even though you have to use oxygen too. It's great that you realize how important exercise is!
The answer to your questions depends a lot on what your "normal" oxygen saturations are. Another factor is how you feel when you're oxygen is beginning to get lower - you are probably a very good judge of how much you can handle.
I would recommend that you ask your doctor how he/she feels is "safe" for you. Each individual is different so you should go with whatever your doctor is comfortable with.
Best,
Lyn (site moderator)