According to the Limitations section of the Learjet Model 35/36 Aircraft Flight Manual (AFM), flight crew and passenger oxygen masks are not approved for use above 40,000 feet cabin altitude.11 A "warning" in this section states that "passenger masks are intended for use during an emergency descent to an altitude not requiring supplemental oxygen." The manual also indicates that "passenger masks will not provide sufficient oxygen for prolonged operation above 34,000 feet cabin altitude. Prolonged operation above 25,000 feet cabin altitude with passengers on board is not recommended." In addition, the manual indicates that, above FL 250 (Flight Level 25,000 feet), in aircraft with ZMR-series oxygen masks, one flight crewmember must wear the oxygen mask around the neck; in aircraft with 6600214-series oxygen masks, the masks must be in the quick donning position.12 Further, the manual indicates that, above FL 410, the pilot, copilot, and passengers must wear oxygen masks. The maximum operating altitude for the airplane is 45,000 feet.http://www.ntsb.gov/investigations/fulltext/AAB0001.html...
12 In accordance with 14 CFR 25.1477(c)(2), flight crewmembers must be able to don the oxygen mask within 5 seconds for the mask to be considered quick donning.
That corroborates first person statements from a good friend of mine who flew THAT aircraft type.
He hated the rubber jungle.