Yesterday I met up with @WannFly and we did a trial run to get more prepared for a potential trip out to Wyoming this coming fall. It was great having another pilot along for this test as the highest I had been before was 11,000 and this upcoming flight might need higher and thus 02 as well. All data is for a 1972 Cessna 182P (stock 230HP). We took off from 1450msl at around 75F and DA=2600. The plane was loaded similar to what we will fly. The takeoff roll was nice at about 1000ft with some headwind. There was a very sparse layer of FEW @5500 so no problem going VFR up around/over those. For this one I flew at 85mph which is recommend for best rate of climb above 10,000. From the ground we were climbing around 1000fpm and held 800fpm to about 7,500. Then the climb rate started dropping to around 550fpm and would continue down to about 0fpm at 14,000. We started with manifold pressure of 23" and was at 9" when we leveled off - didn't expect it to be so low. So we stopped there...but for more than one reason. Temperatures were a bit weird in that they remained the same from 6,000ft to 11,000ft so it was warmer than the standard lapse rate. The field altimeter was 30.10 This was also the first time I have worked out the engine HP loss based on our altitudes. At 14,000msl we were at DA=15,200ft. The 230HP engine will put out only 125HP at that altitude...wow!!!! In hindsight, its funny to think a fatty Skylane with a fatty pilot can even fly at 125HP. But we both agreed that most important lesson learned was O2 management. We had a single oximeter and for this test and probably should have each had our own. It displays heart rate and O2 and once up at the higher altitudes it was easy to mistake one for the other. Later I figured out a different display option I should have used. We were both showing in the low 90's around DA=10,900. Then after that it started falling off quicker than we had expected. We started seeing numbers in the low 80's (eg 82 & 83) when at DA=12,100ft....DA=15,212ft. What was more surprising was how quickly those numbers would drop after taking 3 or 4 hits off the 02 cans. I thought a good 3 or 4 hits might last like 10 minutes but I think it was maybe lasting 3-4 minutes. At one point I could start to tell, feeling a bit weak, but he was throwing me a few random math problems and says I didn't screw up any. Now I know what it feels like. I am also a scuba diver. It was not like being narc'd - that is a bit more euphoric. This was a bit more "low energy" which of course makes sense. In either case we decided since we were using more 02 than we initially guessed that we should head back down and get some fresh air. We spiraled back down which also takes a while. I was in a bit of hurry as I still had one flight left after this. So our ears weren't happy with that descent. On the ground we chatted for about 15 minutes, made sure neither of us were feeling weird and were both still just as stupid and then he drove home and I flew home. I believe the 182 has a service ceiling of 17,700. Had our day been about 20deg cooler and had the temperature lapse rate been more normal I think we could have reached a DA=17,000 or just a bit less. So fairly close to book numbers. Thanks for riding with @WannFly! Hopefully your annual wraps up soon so you can get back to flying your baby!