Ladies and Gentlemen, some of these posts are treading the line of rule 1. Let's please ratchet down the snarkiness a bit.
I think this is symptomatic of the fact that dogma and sales pitches are oft used in place of any hard facts when it comes to fretboard treatments. Usually you have one camp that has the $3 Jim Dunlop lemon oil solution, the next camp has the $$$ Fret Doctor bore oil solution, the third a $free solution using something out the store cupboard, etc.
Youtube doesn't make this any easier. I saw a video recently of a pawn shop bought Squier Affinity Strat being 'upgraded'. Cue some dude wailing away on the fretboard and frets with steel wool, and then slopping on some sort of oil to 'hydrate' the fretboard. He probably learned his tricks of the trade from Internet forums.
And ultimately, what do people want? A fretboard that won't crack or show any signs of damage over time, or a dark fretboard that looks like a quality slab of rosewood of the sort not used on production line instruments since the '60s and '70s? What are people
really chasing here? Is this really a lot of fraught conversation about a fairly trivial corner of instrument care, or is this an aesthetic concern? Do people want Pau Ferro to look like '60s pre-CBS rosewood boards perhaps?
I've not been clear enough in this thread. I like a darker looking fretboard with a subtle low-level glow and radiance to it. I've recently sorted out a dry-looking and dirty fretboard on my Epiphone Wildkat. It looked like this:
Sort of greasy in the popular spots (the previous dude loved first position Cowboy chords), but you can see dry looking wood near the frets. I wager this board probably didn't need anything done to it. It wasn't splitting or cracking, and none of the frets were unseated in any way. I'm not sure what other metrics I could use to determine the condition of the fretboard really. Is it hydrated enough? I don't know! I don't know how I would find out.
But, I don't want the fretboard to look dirty, unevenly greasy or to have that dry quality to it. Ideally I want a nice dark-looking board to make the inlays and binding 'pop', as well as the frets once I've polished them out.
This is after a few light applications of BLO and Danish oil, thinned out with mineral spirits. The mineral spirits help the oil penetrate a wee bit (from some fairly non scientifically robust tests on scraps of wood), and ensure that the oil either goes where it wants to go, evaporates, or is easy to remove. I use a very small quantity of oil overall. I remove excess after ten minutes or so. The wood may further barf out a bit more oil as well, so I always check for this. I then finish with a wood wax and buff.
This is because I want a discrete finish on the fretboards of my instruments. I like the feel and the look. I'm not worried about the welfare of the wood as such, though I imagine a layer of dried oil-derived finish and wood wax will hinder moisture transfer, either way.
Ultimately this is how I like to do it. This isn't necessarily how anybody else should do it.