My preference is Tung Oil.
Wipe it on, let it set for 5 mins wipe again with a dry rag.
Wait an hour and buff it up.
http://en.wikipedia.org/wiki/Tung_oil
Tung oil can be re-applied yearly or as necessary.
One of my old set-up guys used to steel wool fretboards
with 0000 steel wool, wipe on the tung oil quickly, wipe it off
right away,and it gave the fingerboard a nice feel, fast but not too slick.
Good stuff.