Rosewood is a wood that grows in Brazil and India. It has a very think resin in the wood that protects it. This resin is the reason why they use rosewood to make guitar fret boards, bass guitar fret boards and bridges. There is no need to put any type of finish on your guitar fret board. Especially thick oils like linseed oil that will leave a sticky residue on your fret board. Oil finishes will only collect dirt. The resin in rosewood protects the wood better than any finish that could be applied.