I've been playing the classical guitar for 40+ years and have never used any polishing or cleaning products on them and they have survived pretty well (better than I have!).
Fingerboard: I use a moist cloth (crosswise) to clean it when I change strings, then dry any residual moisture with a piece of kitchen towel. If the fretboard gets particularly grotty I use a moist cloth and a moist soft toothbrush. (When I say "moist" I mean "moist", not wet.)
Body of the guitar: I dust with a clean soft cotton cloth [edited, from micro-fibre], if necessary I additionally clean with a (different) clean, moist soft cotton cloth [edited].
The only exception to the above is that I apply some protection to the body that get most wear, e.g. the top of the lower bout, where the arm rests. In those places I periodically use "Renaissance Micro Crystalline Wax". It was developed by the British Museum for use in conservation and it seems to be gaining acceptance in the violin world as a way of protecting the polish where fingers touch.
People sell all sorts of fluids, etc for guitars - whether they are actually needed by most people is a moot point.
Last edited by JohnB on Mon Jan 08, 2018 12:13 pm, edited 1 time in total.
Hermanos Conde 1968, Stephen Frith 2007 "Guijoso"