I went to 510 during an air test recently and the aircraft got there with ease with over 10,000lbs of fuel leaving 470. Initially we arrived at at 510 with a speed of 0.77 and slowly accelerated to 0.8. The high speed and low speed tapes are at 198 and 158 at that altitude and 0.77 was approximately 180 indicated. Oxygen was at 97% at the start of the flight and either I or the co pilot or both had the mask on in normal for around 2 hours and the oxygen was 96%when we parked. I attribute this difference to the bottle getting cold rather than the oxygen actually being consumed. I am doing a fairly long flight in a few days and I will sit there doing the darth as high as practical (probably 490) for as long as I can stand it and report back. My gut feeling is that it won't consume any oxygen and will make drinking tea very difficult.