Mansfield,
Great points, my thoughts exactly as I read this thread. I ensure my co-pilots wear the mask when I leave the cockpit - how? I always let them use the Lav first. They then see that I have put my mask on before they leave and know I expect them to do the same (they find the antiseptic swabs nearby when they come back)
I can't say how many FO's: fumbled putting the mask on, fumbled setting up the audio controls for intra cockpit communication and fumbled ATC transmissions. Twice in the last 20 years, last time about 2 years ago I had to put the mask on for real - cabin pressure loss and smoke. It wasn't easy-peasy but it wasn't the horrendous distraction it could have been.
Captains lead by example, Captains mentor by example. That young fellow or young lady in your right seat is going to fly you around some day as the Captain.