"Seriously, don't put anything oily/waxy on the fingeroard, or anything that can build up and clog the strings."

Actually, lemon oil is the preferred item to put on fingerboards, and is recommended and used by many, many players. Other oils, yes, they could build up and gunk up and all those things, but lemon oil doesn't.

Now, you'd use it maybe twice a year, three times tops (but only in dry climes). Used weekly it would, indeed, gunk up any instrument.

The wood used for fingerboards eventually dries out, particularly at the upper end of the board, where your fingers aren't adding their own natural oils. Dried boards give rise to cracks and splits, or what's called "fret sprout," where the board shrinks and the frets, being metal, don't, so the ends stick out in deadly sharpness.

Bob Clayton