......Seriously I have never had to grease / replace a fill nipple. As long as you have a cover on it, keep it dry and leave some air in your tank. You will be fine.
I had a little leak probably due to low low tank pressure
so I checked the oring and it was a little pinched so I turned it upside down so that side of the oring wouldnt be the sealing side (if that makes sense)
after consulting various sources I played it safe and left it dry