Planet Waves PW-LMN Lemon Oil, natural cleaner and conditioner, protects your guitar, removes dirt, grease, and wax build up, helps condition your instrument to resist dryness, extends the life of instruments, recommended for use on unfinished woods, 59ml
All in highly recommended and well worth buying for any guitarist with a rosewood boarded guitar.
I'm a tech and this is all I ever use on my own guitars and all my customers guitars. It properly nourishes dry boards with a few applications. (apply, wipe over and remove the excess and then repeat until satisfied). One application is often enough.
The price is great and no real downsides other than the cap can get a bit loose if you overtighten it.
I bought this lemon oil to revitalise a rosewood fingerboard that had completely dried out on a guitar I recently purchased second-hand. The results are impressive. The oil both cleans and hydrates the wood and the fingerboard becomes much more comfortable to play after application.
I would advise removing the strings and giving the fretboard a quick wipe down first. Applying the oil is very straightforward (I used a dry, new dishcloth) and a little goes a long way. I expect this bottle to last me many years.