3/8" x 9/16" #8 O ring.
Propane guy tells me there is no difference between the propane compound and plumbing compound.
Your fitting will leak when these dry rot.
Best to have a few on hand.
I taped a couple inside the case of my porta heaters so to have it handy.
I also gave them a very light sheen of silicone grease to help with dry rot.
(Same grease used on spark plug boots.)
FYI - I searched this part number and found it on Amazon (from mr heater) for $6.xx shipped for the 20-pack, vs paying $13+ in shipping charges to buy from mr heater directly.