Wow, this bike certainly has seen his share of hi and low temps...
Fine sanding of all sides of moving parts to restore the smooth surface. Do not sand so much that it would change clearances.
I found some teflon lube. Rather tacky. I though of chain lube too. But at the same time I was worried that fuel would just wash it away anyway, an only interfere with fuel filter or worse, injectors. So if it isn't designed/documented to lube the gas cap, that's because it doesn't need it.
At most, wd40 can burn so that's the most I would use.
The cap does need fuel and splashing of it, to move water away.
I fill up about 1/2 inch above the bottom of the hole, all the time, so the cap soaks in fuel a little while.
But that is not for leaving in the sun or over winter indoor. Fuel would expand and spill. Only do that when you know you'll ride another 5-10.
Of course, inspect at every fuel up for deterioration.
Oh, and be sure to leave that tiny pinhole clear after your cleaning.