I usually use 0000-grade steel wool for my unfinished ebony and rosewood fingerboards. This fine grade stuff will get the gunk off the board in no time and it won't damage the wood. You do need to cover the pickups with some "detacked" Scotch-type tape (put your scotch tape on your shirt so some of the fabric gets on it, then put it on your pickup so as to avoid tape residue on your pickups), otherwise you're going to be stuck with steel wool shavings stuck to your pickups. After that, I spray lemon oil on it then wipe off the excess. Fortunately, I only have to do this ritual every six months which is recommended, especially if you live near the beach, like I do.
BUT, it should be said now that this steel wool is NOT recommended for fretboards that have glossy finishes, i.e. Bryant's lacquered maple fretboard.