I lubricate them with "Triflow." It's a silicone lube sold at bicycle shops. It does not attract dirt. If you put a rag around the valve and pump a bunch if it around and through the threads, it floats all the dirt off them.
Tri-flow on the aluminum threads of the valve only, not in or on the boat. 303 is sticky when old. My 1980 18' Avon Spirit is good as new at 35 years old, not even a patch. She will run Cat in two weeks and the Grand after that. Tri-flow is not ruining anything.
Plus one on the Tri-Flow or similar. If they're really giving you problems you might consider taking them out for a good cleaning. I did that on my old pig and it was just what the doctor ordered. All I had to do was remove the screws then gentle the valves out with a pair of judiciously applied channel locks. Just be sure to put them back exactly the way they came out or they won't seal properly.
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