I use Ox-Gard from Lowes, but only because that's what they sell. Noalox is good stuff also, but it's messier if you get it on your hands. Ox-Gard wipes right off without a stain. There's also a silicone product called Nyogel that's clear and does the job well, also. Its a gel that is designed to prevent oxidation without being conductive. It was the choice of our engineering department for preventing oxidation on low voltage contacts and 220VAC relay contacts as well as AC connections that had history of heat and arc related failure. Air is your enemy with electrical connections.
Less is more when you use any of that stuff. Cover the threads with as little as it takes to put a light coat. It's good enough to dip the tip of a Q-tip in the chemical and hold it on the threads while you turn the device, then wipe it down lightly with a towel. It's not necessary to glob it on.