I have been using D'Addario's Lemon Oil for the past four years and I have to say that it never disappoint me. One bottle usually last me for about two years. Excellent for condtioning dehydrated fretboard. If you have open wood dark fretboard like rosewood, Pau Ferro, etc I suggest you apply it once a month, in a light coating. Do not use it frequently or apply excessively as it will over saturate your fretboard which will damage your guitar neck. In the past I had apply it on a satin finished maple fretboard, it did work perfectly, but I am not so sure about its long term application, it might darkened the colour of the maple wood if apply for a long time, so be cautious.
If you have a polyurethane finished (glossy finished) fretboard, you can use this to clean your strings, to avoid rusted strings. I wiped the strings with the lemon oil after each play, this significantly extended the string's life, and a great alternative to string cleaners like GHS Fast Fret and Fender Speed Slick, which are overpriced.