My '73 3c had one of the socks already damaged from the factory, hadn't been inserted quite right and was squashed asymmetrically. I only pulled them out a couple of times, but I seem to remember greasing liberally with Vaseline petroleum jelly (no jokes about 120s, please, have some respect). Even the busted one seemed to seal, but how can you really be sure if you don't have a little leak?
Ken