Years ago I used "lemon oil"...i have since repented of my ways, and will not let ANYTHING with petroleum distillates touch my fretboards anymore. There may be other products, but I have had great success with Fret Doctor. I had a LP Special (2004 model, not a vintage guitar) that I picked up years ago that had a very ugly, dry fretboard...it really came to life after using Fret Doctor. Like a whole different piece of wood. Only once or twice a year application. Let it soak in. The wood will not take-in what it doesn't need. YMMV...