I use Deoxit and it works great. I put it on and leave it sit for a while and then "polish" it off. It doesn't make the sound any better, IMO, but it definitely works as a contact cleaner. It has helped me out in flash lights, phone batteries, and an old 70s receiver. The key is, use only as much as you need and wipe excess off. I use the brush on type, not the spray type. The tiny little "fingernail polish" bottle has lasted me quite some time as well, as you don't really need much. I haven't tried it on any stubborn RCAs though.
Dielectric grease is theoretically supposed to be pushed out of the way by the contacting metal surfaces so that there is still contact, then the grease prevents oxides from getting into that contact area by filling in the gap around the contact area. I use it on my spark plug wires but not much else.
