"I was under the impression that the reason cannula were 'required above 18000 feet was an issue with excessively low CO2 if you didn't re-breath some of the exhaled air."
You are absolutely right, I used to frequently work with a (recently retired) consultant anaesthetist who was a keen pilot and very much up on high altitude physiology.
His advice was that nasal cannulae are fine at low(ish) levels but that the rebreathing bag that you get with the mask makes a big difference to CO2 levels and some improvement in O2 sats as you climb higher.
Personally, I just stick with masks (and carry a sats probe). The masks go on at around FL100 and stay on until I pass back down through FL100 again. I say "around" as density altitude can make 8000' feel like 12500' so adjust as required.
Advantage of masks - inbuilt rebreathing bag helps with CO2 and O2 levels when you are higher.
Downside of masks - sweaty face and poor seal with mic poked inside. Using the mask with inbuilt mike makes u sound like a constipated dalek in an echoey room.
Advantage of cannulae - won't spoil your makeup. Less burdensome.
Downside on cannulae - better make sure you don't share them if you reuse them!! YUK.
Chacun a son gout as the French rugby team might say (amongst other things!
),
SB