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Should I oil my fretboard?
Most fretboards are unfinished, so you have raw wood exposed to the air, moisture (or lack of it), and whatever is on your fingers (sweat, oils, ketchup, beer, lotions, boogers, ...you name it). Most guitarists I know use some sort of product to clean and condition their fretboards.
Everyone seems to have an opinion about what, when, and how to condition fretboards, and now you get to hear mine. I am not basing my comments on scientific research, but rather 40+ years of owning dozens of guitars and working on many more. Over the years I've put just about everything imaginable on my fretboards; olive oil, linseed oil, WD-40, mineral oil, various furniture products, and various products made specifically for the purpose from companies like Dunlop, Martin, and Gibson.
My conclusion? I can't think of a single guitar that has been ruined because I or somebody else put the wrong stuff on it.
Now that's not to say that they are all the same, and I'm sure there are plenty of substances that definitely shouldn't go on a fretboard. I don't think I'd use olive oil, linseed oil, waxy furniture polish, and WD-40 contains some solvents that could interact with some glues and finishes.
Before going any further, why put anything on a fretboard? Several reasons; bring out the beauty of the wood, darken the wood, and to preserve the wood. But one of the more common reasons given is to moisturize the wood and keep it from drying out. Well, this opens up a whole other can of worms, because your fretboard is probably not the only bare wood on your guitar, especially acoustic guitars. Dried out wood can cause all sorts of problems; warping, cracking, shrinking.
So is oil the best way to moisturize wood? I don't think so. In fact if the wood is too dry, applying oil will be a barrier to keep moisture out. The best way to moisturize wood is by keeping it in an environment with the proper humidity, which is 40-50%. If you do this you pretty much eliminate the reason to oil it. You can do this with in-case humidifiers, room humidifiers, or whole house humidifiers. High humidity isn't good, but low humidity can ruin a guitar.
There are products that are presented as "lemon oil," and folks ask about them. I'm not aware of any product the is pure lemon oil. Most are lemon scented, or at best have small amounts of lemon oil in a mineral oil base
I still occasionally oil my fretboards, but not nearly as much as I used to. Maybe once a year. I've had my Les Paul for four years and never done anything to it. If you do want to oil or condition your fretboards, products made especially for the purpose are probably a safe bet. Lots of people swear by Fret Doctor. I think you're also pretty safe with mineral oil based products or pure mineral oil (yes, the same stuff grandma uses to get her bowels going).