Many of these stories told here seem to indicate that flying at altitudes in the 'teens' is ok... Well, it's a dangerous game you're playing, as your performance level is almost certainly not 'fine'. The lack of oxygen would also cause an element of euphoria that is unlikely to be noticed, preventing you from realising any performance shortcomings.
Even flying at 8000 reduces performance. Eyesight is significantly affected by hypoxia (even at that level)... try taking a gulp of pure oxygen after 3 hours at 8000 at night - you'll be suprised.
Airliner warning systems provide a 'hard' warning when the cabin altitude exceeds 10000 feet. The automated passenger oxygen system is activated passing 14000 feet. Above 45000 when pressurised it is a requirement in many airlines and countries to have the oxygen mask sitting around the pilot flying's neck - because the time of useful consciousness is so short, there's too much risk in not having it 'at the ready'.
northwing, time of useful consciousness isn't a legal term, it's a medical one...
Sure, walk up a mountain without the stuff, but flying is even more isolated...
Lancer