A dirty, uncared for guitar fretboard is a sight to be avoided, but many guitar players clean their guitar fretboard too much. The dirt that builds up is old grease and dead skin cells which is not a good look if you pass your guitar to someone else to play.
The fretboard of you guitar is made from a hardwood that is meant to stand up to all the punishment your fingers and your storage and transport methods can hand out. Dark woods are used for the front of the neck so that marks form oily fingers don’t show up. It is especially chosen to resist abrasions. A finish applied to the wood in the factory would wear out from use very quickly so when you get your guitar, the neck is unfinished.
If you have a cheap guitar, you might find that your neck looks blotchy. This is because some of the cheaper guitars are painted with a coating to make the look nice and shiny. These kinds of finishes are just window-dressing and don’t last longer than a year or two.
A guitar fretboard absorbs moisture from the air when the weather is humid and the water goes back into the air when the weather is dry. The fretboard is glued onto the guitar neck so the underside is protected fro the action of moisture in the air by the sealing quality of the glue.
So one side of the fretboard is expanding and shrinking because of the moisture and the underside stays stable because it is protected. This can eventually lead to the fretboard warping and cracking and the whole neck is vulnerable to warping.
So we are left with the need to seal the fretboard against the danger of warping and cracking. There is a range of preparations made for the purpose of protecting the fretboard. Most of these should be avoided. The general rule for looking after a guitar fretboard is to clean it when it is dirty. Never use polish of any kind. A little liquid soap will clean grimy areas and if your fretboard looks dull, use some kind of fretboard cleaner like Formby’s or Fret Doctor.
Some preparations like lemon oil will leave a layer that attracts more dirt. Don’t clean your fretboard until you can see the dirt. For most people this will mean about once a year. Use a “magic eraser” type cleaning pad and perhaps a little naptha on it.