The ICAO standard atmospheric pressure at 12,500' is 9.168 PSI. From this subtract the maximum cabin differential pressure of 5 PSI to get a minimum atmospheric pressure of 4.168 PSI which roughly correlates to the pressure at FL310.

However FAR 91.2911 specifies that you may not fly "(b)(i) At flight altitudes above flight level 250 unless at least a 10-minute supply of supplemental oxygen."

I have a supplemental O2 system in my IV-P and durign flight testing, back before RVSM, I flew to FL300 and one time returning from OSH I flew to FL290 (dodging towering cumulus over CO). In '96 I flew the Denver to OSH race at FL280 and 95% power to win the race.

Above FL280 the plane is not "happy" and requires a lot of attention to fly well. The I do most of my flying between FL210 and FL250, leaving the O2 tank in the hanger.

Regards
Brent Regan