on a real '68 p-bass? i want to say nickel. just clean them up with a little guitar polish, you don't want to go removing plating with abrasive metal cleaners. sometimes i use 0000 steel wool, as it seems to clean the metal without really scratching it. follow up with a tiny bit of machine oil on the sliding surfaces, and don't worry so much about the "original luster". nickel "ages", and that's part of its charm.
Try WD40 on a rag, but try not to get it on the lacquer (it'll make refinishing very difficult some day). The idea is to clean the metal with a mild solvent but not to use anything abrasive. You don't want them to look dingy or dirty or smudgy, of course, but at the same time I'd avoid trying to shine them up because you'll have rubbed off some of the patina that helps authenticate the vintage instrument. Mojo. Vibe. The whole "relic" thing. Besides you don't know how many times it's been buffed before or just how much plating is left.
Having said that, if you HAVE to polish it, try NevRDull, it's a wad of cloth in a blue can with an oil and an abrasive impregnated in it. It's gentler than Noxon.
Naptha (Zippo type lighter fluid) does not harm finishes (even nitro), evaporates completely pretty fast so no oily residue and takes off any greasy, sweaty, waxy, etc residue. Whatever that does not get off I would leave as you would be removing plating to some degree with anything even mildly scratchy or abrasive.