I took a gamble and bought a used B+W Circular Polarizer off eBay. The filter looks great but the adjusting ring is a little bit sticky. Is there some way to lubricate this?
Use some Isopropyl alcohol with a cotton swab around the threads. This will help clean them and evaporates without residue. (Safer than water.)
Check the roundness of the filter. Perhaps it's sticking because it's out of round?
You can lubricate with a bit of silicone grease or lithium grease. Both are non-reactive. Only use a very little bit! Start with a quarter of what you think you might need. It's easy to add a little bit more but frustrating to clean up after over-application.
What do you mean with sticky?
I had a Hoya HD circular polarizer that fell apart - I got it back together but it was too lose then and it was exchanged under warranty... so, a little bit on filter design.
Your filter consists of an outer as well as inner piece- these have both got a "grove", the inner piece on the outside, the other piece on the inside. The connection between the inner an outer piece is made by a brass ring with a bit cut out - so you place the ring in the inner component, squeeze together gently and lower into the other ring, it will spring back and lock the pieces in place.
To keep the filter moving smoothly, the inner ring is lubricated - if you lose the grease, the filter will become loose and may for example incur sideways movement (this is what I ended up with on the Hoya when I got it back together.) However because of the application of grease, there will be some stickyness in the system as well - on the other hand it keeps the filter in place and prevent accidental movement.
So, if your filter moves smoothly and is only a bit stiff to move, I would assume it is in perfect working condition. If it does not move smoothly it could be dirt, dents etc.
Having said that: High end filters are actually a bit stiffer than cheaper filters when it comes to turning them.
SEWING MACHINE OIL (get at fabric store like JoAnne Fabrics)
I have 3 B+W CPL filters (purchased from B&H so pretty sure not fakes) and after a few years all became stiff. I sent one back to B+W and they replaced it for free but then a couple of years later it too was stiff.
What I finally did was to use a drop (no more) of sewing machine oil on the seam between the two moving parts. This was a year ago and so far I've had no problem with stiffness or any negative impact on optics.
Hold the filter vertically and put no more than a drop on the seam between the two moving parts. Then hold one side and turn the other, always going in the same direction, to distribute the oil around the entire trace where the two parts rub against each other. You'll have to go around maybe 20 or more times to get the lubricant distributed all the way around. Wipe off any excess oil from metal edges. During entire process make sure that no oil gets on the glass surfaces