I have bought a classic guitar for my nephew who is starting classes. The vendor gave me as a gift a little vial of "lemon oil" (translated from the french) saying just quickly something like "it could come handy for maintaining".
Can someone tell me what it is and it should be used (if ever) ?
It's used for re-moisturizing and preserving the guitar's fretboard. If you don't apply some type of fretboard oil (like lemon oil) every year or so, you'll notice the fretboard getting dried out.
Also, this oil isn't typically used to clean the fretboard, it's just used for moisturizing. You'll want to clean off your fretboard before applying the oil, which can be done using zippo lighter fluid (or Naphtha). Just put some zippo lighter fluid on a rag (avoid any sort of fuzzy rags, as this can lead to cloth fibers getting caught in the frets while cleaning), and clean the entire fretboard. Zippo lighter fluid will also dry out your neck even more, so make sure that you can apply oil to your fretboard shortly after. Also, since lighter fluid is flammable and has such a high evaporation rate, don't use it near any open flame and make sure you are it a well ventilated area.
Hope this helps!
Tip: Do not use too much lemon oil on the fretboard. Three or four DROPS is enough to get decent coverage. Apply a few drop along the neck then rub it in with a clean cloth. Use another dry cloth to wipe off any excess oil.
Make sure the oil doesn't build up against the frets as these will get gummy over time.
As in the question I just answered, here is a link to tons of guitar maintenance and repair information:
It is used for cleaning the guitar's fretboard from the gunk and the dirt and for hydration and protection after cleaning.
