Take coltsfan's advice. It is uber important to keep your O2 sats up, preferably above 91%. The magical number I was told of is 89% though. Anything below lung damage can happen, capillaries can rupture. Whenever I was in the hospital and real sick, my doc's wouldn't let me leave until I was above 91%, which left a tiny bit of cushion. When laying down, it is normal for your sats to drop a bit, so many people only use oxygen at night.
You were talking about sats dropping down to the 50's and 60's. I am wondering if maybe you are mistaking sats for lung function (FEV1), they are two totally different things. Your sats are related to the amount of oxygen your body is absorbing, your PFT's (example FEV1) is a measurement of how much lung capacity and performance you have. You can have significantly diminished lung function and still absorb oxygen readily. One the flip side, a person can actually have good lung function and have bad problems absorbing oxygen, say if they have a bad infection.
Hope this helps.