A special guitar safe blend of lemon oil and cleaner that will protect your fretboard and clean fretboard gunk. This is the same stuff I use here and can personally attest to it's effectivness and safety for instruments of all vintages and types.
 
Spray a little of this elixer on a paper towel or rag and gently rub into the guitars fretboard. Do not spray directly on the guitar as this will create a greasy mess that will then have to be cleaned off of the finish.