In a recent FAA knowledge exam, a question was asked whether medical grade or industrial grade oxygen can be substituted for the aviation grade supplemental oxygen for General Aviation, Part 91 flight. I have looked in the FAR/AIM for answers. But, there is not a specific regulation prohibiting the use of medical grade oxygen in aviation.
The FAA Safety Brochure OK-09-439 recommends that medical grade oxygen should not be used. Yet, there is a Civil Air Patrol document (another government entity that is regulated in part by the FAA) that refutes this due to changes in the understanding and delivery of supplemental medical grade oxygen.
Does anyone have a clear cut and recent regulation or document that clarifies this question? Please include links to the documentation.