For unfinished fingerboards (rosewood and ebony, mostly), use a good quality, oil free 0000 steel wool to clean it, and then lemon oil (I use Kyser Lem-oil, but then I get it cheaper than you can) or mineral oil (which is most of what is in any good lemon oil). Be careful with lemon oils, though, as some brands are mixed with other chemicals which can react badly with the guitar.
What you need to look out for in polish, by the way, is wax and silicon, either of which can cause serious long term problems. The best stuff on the market is called Powerchord (made by BioTerra Industries), though the GHS guitar polish is also just fine.
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