The height limit isn't because engines need oxygen. It's because in order to fly in less dense air, you need to have higher air speed to generate the same amoun

The height limit isn't because engines need oxygen. It's because in order to fly in less dense air, you need to have higher air speed to generate the same amount of lift, and above typical jet cruising altitudes you hit the speed of sound and run into issues. Supersonic aircraft fly at higher altitudes in general - the Concorde cruised at 56000 feet.