Bright Boys, the grey ones are beginning to lose favour with some as they will put scratches in your rail which collects dirt over time.
Find out if your rail is brass, and not the more common stuff now used - just blanked out on what they are called. If your rail is brass, don't use them; you will spend the rest of your life cleaning and cleaning and cleaning. Very few people use brass track now.
The latest fad, and it makes sense, is to use all the stuff out there that is designed to get rid of oxidation on metals, like Brasso, Fitz and Maas, and autosol, etc. Some suggest wiping the track down after with alcohol, others say you don't need to.