Lemon oil is mostly just mineral spirits with a smell. I use it regularly to clean fretboards it's good stuff. However, it will do noting to bring back color to help with fret sprout from fretboard shrinkage and help close open grain from a board being super dry. I use the Howard on antique furniture, it's a good product but won’t address dry wood. I don't know of any other product that will do what Fret Doctor does. It was not produced for guitars originally, it was created by a chemist as a bore oil for restoration of historic woodwind instruments. In particular, revolutionary war era wood fifes. They would dry out and the grain separate, making them unplayable. The guitar part was an accident, but it works.
When I first got my 93 solid Koa Carvin DC 127 it was filthy. It had sat overpriced on the wall at a local Pawn shop for over 5 years. I went to work for the shop and the owner and I cut a deal on it. When I got it home and cleaned it up, the ebony board was so dry the grain had opened on the board. The grain ran longwise up the neck and the grain had opened between several frets could not see it because the board was so encrusted with filth, but it was there. Cleaned it up, polished the frets and then hit it with Fret Doctor. Applied it a number of times over a couple of days and let it soak. You can't over oil as the wood will take what it needs then stop absorbing, and it will just stay on the top. When it stared doing this I wiped it down and took a look. No open grain, nothing. That is exactly what this stuff was designed to do in restoration of antique woodwind instruments. Have been playing that guitar out now for over 10 years still no open grain cracks in that board. Show me anything else that could have done that? Not saying it doesn't exist just that I have never seen it. Have been using this for almost 20 years now on my fretboard bridges etc where there is un protected wood. Also use it on my cutting boards to prevent them from cracking and drying out after being used and cleaned, it is 100% safe to use even with food processing on a cutting board.
Some of the other products that folks use are toxic, and that is something that should be considered. After 20 years, I am just now getting into my second bottle. That is only because after using the first bottle for so long, it had dried up a little and started to discolor. Every guitar I own every year or so gets the board cleaned a fret polish and a fresh coat of Fret Doctor.
