12500' and no 02 is no problem. Regularly fly up to this in the US, above 12500 for more than ˝hr you're meant to use 02. Hardly notice it, and I'm by no means the fittest bloke on this planet [and I smoke
] Talked to a ferry pilot who was bringing an aircraft back from Japan to the US via russia, he forgot to switch the 02 supply on, and once airborne he was ******ed as it was behind the fuel bladders. Still, he was up to FL190 due to the MEA's en-route and the only major side effect he reported was a blinding headache when he landed....
Cheers
EA