Just learned something from a somewhat stiff non-vernier tuning cap this weekend---there is evidently some type of hard film that sometimes forms on bearings & "race" surfaces which is not easy or fast to remove...
You may have to remove that cap and flush out old grease & residue with acetone like I did---had to "dribble" acetone into bearing cavity, rotate shaft back & forth quickly, blow-out, & repeat until smooth operation was achieved... Then repack with a quality grease.
Took a long time & many repetitions to get mine back to normal, but worth the trouble...
Imagine it will be much more difficult with the 500's small vernier drive cap, but worth a try !
Do we know for sure there is nothing plastic in the drive? I was a little hesitant to put anything too aggressive in there for fear of some plastic melting. I did try lacquer thinner in the drive from a cap from a Royal 755 and it didn't seem to do anything, but I can't say I spent a lot of time with repeated applications. I will try again with more patience. I need INSTANT GRATIFICATION when fixing my 50-year-old radios.
If there is no plastic, I might have a solution (no pun intended). We clean baked-on varnish/oil residue from model airplane engine parts by putting them in a crock pot, on high, filled with good old-fashioned anti-freeze. It's miraculously effective overnight with no damage to aluminum or steel parts, but it's very hard on plastic. It also loosens them up even if they are glued together with castor oil from 1950. Using it to make beef stew afterwards is discouraged, of course.
BTW, I don't know if anyone has tried this, but Air Tool Oil works very well to lube them afterward, and it appears to have no tendency to gum up over time, unlike most household oils (3-in-1, Marvel Mystery Oil, etc.). And prevents rust.