Fender maple fretboard and Adamas walnut fretboards are sealed and finished. But haven't heard that about Rosewood. My luthier is opposed to oiling the fretboard as he feels it leads to build up in the pores that is difficult to clean and may even void warranty on instruments he builds. As for plant based oils I hear they can go rancid so am surprised about linseed recommendation. I have used mineral oil and thought it did good job.