Aviators Breathing Oxygen, Medical O2, and O2 used for welding/cutting, ARE EXACTLY THE SAME! It's all boiled LOX out of the same tank, with absolutely no moisture of other contaminates.
I know what your CFI said, and the rules clearly state that only ABO may be used in certified aircraft systems. However, there is no longer any difference between the three "types" other than price.
As with 90% of "airworthy" products the only difference is the paper trail.
There is a loophole that several pilots have/are useing to use welding O2 in their airplanes.
AC 65-9A (Airframe And Powerplant Mechanics General Handbook) states
"Only oxygen marked "Aviators Breathing Oxygen" which meets Federal Specification BB-0-925a Grade A or equivalent may be used in aircraft breathing oxygen systems."
Note the key words "or equivalent"
Read the following article
http://www.avweb.com/news/pelican/182079-1.html
Now, I'm not sure about the difference in aircraft and medical bottles, there may be some difference re: crashworthines there.