I always clear the fingerboard and put lemon oil on it when changing strings, but I only change strings every 4-5 months. I dont want to take down the strings (since you cant put back used strings afterward I guess ???), but there is that "dirt" building up on the fingerboard, and I want to clean it. How should I do it without doing any damage to the guitar?
you kinda shouldn't.
If you keep the strings with some length, you can boil them (yes, boil them) while you clean the fretboard.
It will loosen the dead skin, and "fresh" up the strings.
The guy who does Frets.Com recommends cleaning the fretboard (if necessary) when you change strings.
Sounds like perhaps you need a set of strings if you're thinking about boiling the old ones...

Also, he does not recommend lemon oil, though he says it's unlikely to do any harm. He uses very fine 0000 steel wool and occasionally a tiny bit of clear mineral oil.
Just a soft clean lint free cloth to wipe down the fretboard and strings after each session is all that's really needed until it's time to change strings. For what it's worth, 4-5 months is a fairly long time to go before changing strings. If you play every day, 1-1 1/2 months is about all strings are good for. Some pros change them weekly or even as often as every other day if they are doing serious studio work and need that same exact fresh string tone over and over again.
just clean when you re string. if it is that built up with gunk then you arent changing strings enough.
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Well, technically it could be done, but only in the same way that you could change a cat into a hamburger. It's an unpleasant process, and nobody is happy with the result.