Thanks a lot for posting the answer from your friend, Stiebler
U-boats definitely carried compressed air and pure oxygen. See, e.g., here:
http://www.uboatarchive.net/DesignStudiesTypeIXC.htm
Also the original VIIC handbook in German gives detailed information about the amount of pure oxygen and compressed air.
It's amazing that there is hardly any detailed information available

It seems one has to find a LI/Chief Engineer who is still alive and remembers the actual procedures (measuring CO2 and O2, regulating the ventilation system,...)
Regards, LGN1
Stiebler, from your friend's reply you can see what were the reasons why h.sie implemented the modified oxygen consumption when ordering 'silent running'. It's done to simulate the resting crew.