Fret Doctor (for rosewood fretboards). I stopped using any petroleum distillate-based products (like "Lemon Oil") years ago, and Fret Doctor works amazingly well without the risk of over-saturating the wood. The wood will only absorb what it needs. It was originally formulated for woodwind instruments to keep the woods from cracking.
Many years ago I'd purchased a LesPaul Special that came from the factory as a great player, but with butt ugly fretboard wood- almost greenish in hue. This was a TV Yellow guitar, btw. The wood was dry. Anyways, I purchased it, took it home & removed the strings and treated it with Fret Doctor. It took two applications, but that guitar when I was done looked like it had a completely different piece of fretboard wood. And it played phenomenally well.