I'm surprised no one has mentioned a pulse oximeter. Not cheap (£260) but I wouldn't fly at > FL100 without one.
Or $245 from
http://portablenebs.com/nonin9500.htm
I find it's only when you put the oxygen on and you start to feel much sharper that you realise the effect the lack of oxygen has been having on you. On a flight I did a couple of weeks ago, with the climb and vectors through the London TMA, I didn't get a chance to don the oxygen until FL110 (N-reg). Even at that level, as soon as I got the oxygen on, I really noticed a difference.
I also feel a lot less drained after long flights if I've been on oxygen. Long exposure to even moderately high altitudes can have a significant effect.
It's also worth noting that (in my experience) your oxygen level isn't just a factor of the altitude you fly at. Additional factors, such as starting early and missing breakfast, also have a physical effect on your oxygen saturation readings.
The thing is that FL140/160 is needed only over mountains, so that is another aspect of flight planning.
Or to remain above a band of icing.
There is just no way to do 10hrs with 4 POB at FL140 with anything that's remotely portable. Unless, perhaps, one spends $1000s on the new electronic control kit.
There's no need to go to that expense. Just turn the passengers' oxygen flow right down
