Most fretboard "oil" is naptha based, which is great for cleaning when the thing gets dirty, but does nothing to "oil" the fretboard, since naptha is dry cleaning stuff. It may bring a little oil to the surface, or leave a bit of residue, but it's not like you're going to use it to actually add anything significant to the wood. In fact, you can't really add anything significant to wood, except junk to sit on the surface, or other stuff to completely seal the surface.
Some people think "rosewood oil" is the thing to use, but it's a different species altogether from what's used in guitar making. It's a relative of pine, and it's actually got the same active ingredient as turpentine. And turpentine is actually something that tends to break the cellulose cells down in wood. It's not actually good for wood.
I'm personally convinced that the less surface goo on a fingerboard the better, and that most rosewood doesn't benefit from, or need it very often. PRS recommends using a furniture polish to seal the board after cleaning somewhere on their product site.
Our hands do add a lot of oil to the surface of a fingerboard. I've got a 47 year old SG Special that's never been oiled, hardly cared for, and the fingerboard still looks great.
Anyway my strong feeling is that sometimes in our obsession to maintain, we can do more harm than good!
