Reel Oil (Fishing), Trombone Valve Oil, 3 in 1, anything of the sort is fine, just avoid WD40. Vaseline will attract too much dirt and won't do much to penetrate, avoid it.
As far as the metal-to-wood part, I'd liberally mark the brass with beeswax. You can get it at any sewing store.
FYI, TalkBass peeps like to suggest cat urine for almost any application..
