The recommendations I have seen for O2 flying at c FL250 seems to suggest hyper-alertness to pins and needles, headache, numbness and a frequent check on your pulse oximeter. I too have found that talking to ATC puts a drain on the O2 sat and have a basic mask with the mic penetrating so that I can keep breathing when talking.
FL190/FL220 seems much more manageable and you are that much closer to air if something goes wrong on the O2 delivery.
I don't know about the Mooney Bravo, but the TNIO-550 in the Bonanza (a broadly similar configuration) will maintain 30 in MP all the way to the certified ceiling of FL250 as long as you are turning more than 2500/2600 rpm. In any event, he almost surely had planned the flight at, or consciously climbed to FL250 and then had a problem.