These comments are about the law, not what is smart.

WRT the 18,000 foot limit for using a nasal canula, the only rule I can find that says a mask is required above 18,000 feet is FAR 23.1447.  Part 23 does not apply to aircraft with experimental certificates, see 23.1.  So, legally, it seems, you can use a canula anytime.

Does anyone know another regulation, that applies to experimental A/C, and requires a mask above 18,000 feet?  If you do, please give the reference.

Again, the question is what is legal, not what is smart.

Tom Thibault
L360, 1,000+ hours