A CE-560XL Captain reported contaminated crew O2 discovered during a preflight check.

Narrative

During the preflight oxygen mask test an acidic or irritating sensation was felt in the nose and throat. After breathing the oxygen for 15-30 seconds; a deep inhale of oxygen caused an uncontrollable gag reflex and cough. The other crewmember confirmed the same sensation with the other mask. Approximately 5 minutes afterwards; while breathing ambient air; both crewmembers found it impossible to inhale fully without coughing. Symptoms improved slowly over the next 15 minutes and disappeared completely after 1 hour. Masks were clean and dry and no actual smell was noticed. I would like to make note of the fact that when this was reported to the Assistant Chief Pilot on duty he seemed very unconcerned about it. He said 'well; I guess if you want to down the plane then that's all we can do'. I would expect a person in his position to IMMEDIATELY support the crew and state that the aircraft is grounded due to a problem with such a critical piece of equipment. He acted as though we were inconveniencing him. Crews need to actually TEST the masks by breathing with them during preflight. This problem was likely present since the last time the oxygen was serviced. Management on duty needs to take a problem with a piece of lifesaving equipment seriously and not worry about the potential impact on the schedule.