I need to oil and clean my guitar fretboard and was wondering if a damp cloth and olive oil is good for this?
Short answer: no. As a non-drying vegetable oil, it will eventually become rancid and not be fun. Same as safflower, peanut, sunflower, coconut, palm, etc.
A better choice is a "drying oil" such as linseed oil, walnut oil, or a non-organic mineral oil or tung oil.
Some folks have had good results using a citrus oil (orange, lemon) to clean, and then an application of mineral oil to condition.
Olive oil, or any other vegetable-based oil, is not recommended for oiling any wood as it may go bad, or rancid, after a while. Most commercially available fretboard oils use mineral oil as their main ingredient. Mineral oil is inexpensive, will not go bad, and is readily available at most pharmacies. So if you want a cheap and virtually identical alternative to commercial fretboard oil, try using mineral oil instead of olive oil.
TL;DR You might safely use olive oil if you take care to carefully wipe any excess after application.
Note: this answer is basically an extended version of a comment that I published here.
For what it's worth, I have been doing it for years on several instruments and never noticed any commonly cited downside of olive oil - i.e. rust on the frets, rancid smell, stickiness, collecting dirt, or wood / vernish degradation, and so forth and so on. I also sometimes used lemon oil and could not notice any difference between the two (apart from the smell when applying it, arguably).
Here is what I would advise for neck restoration / maintenance :
- Neck cleaning: carefully clean the neck first with mild soap on a sponge or microfiber to remove any dirt (use as little soap as possible, usually, I put only a drop of soap in a basin of lukewarm water). Then, clean off soap residues with the same humid tissue soaked in water (without soap). You should carefully squeeze the tissue before each application on the guitar neck, the point is that it should just be barely humid so as to help you remove the dirt or soap traces, you do not want to soak your guitar's neck with water!
- Oil application: then, soak the microfiber with oil (just put the microfiber on the bottle opening and turn the bottle upside-down quickly) and rub the neck with it. Be careful to apply the oil everywhere, even in the corners of the cells. Do not hesitate to put oil in excess and it is no problem if the neck is still glossy at this step. Wait a few minutes.
- Excess oil removal: after a while, take a clean microfiber and wipe away any excess oil by energetically rubbing the neck with the clean and dry microfiber. Possibly repeat this step after a few hours. Ideally your neck should look dry (i.e. not glossy, just like ordinary wood), and feel very soft and slippery under the fingers, but without letting any residue on your finger (your fingers should not get oily!).
As for the oil choice, you can read in many places that olive oil is terrible (either in the comments of the present question or here for instance) and that lemon or linseed oil are better choices, and mineral oil even better. Alas, all these affirmation are not supported by scientific evidences (and sometimes not even by testimony), but are more often than not only declaimed as mythical truths.
I know that it is a very weak evidence (on the scale of scientific evidences, which goes from testimony up to review and meta-analysis through pair-reviewed publications), but at least two of us - u/ManOfTwoVisions and I - did it for years on several instruments and never noticed any downside...
Additionally, I might add that I personally prefer lemon oil for the smell, and because it impregnates the wood better, maybe because of its lower viscosity compared to olive oil. Still, when you are out of home or need to quickly restore / clean an instrument, olive oil and the above-described technique is definitely a lifesaver.
I made a quick search on Scholar on the influence of the type of oil on wood coating and did not find many results. The main area of research on the topic seems to be for anti bio-fouling - i.e. contamination of outdoor wood by fungi. I did find one publication, which seems to indicate that stand linseed oil (polymerized linseed oil) did not lead to biofilm formation, contrary to olive oil or raw linseed oil. However, the use case studied by the authors is quite far away from using a fretboard indoor so extrapolating their conclusion seems quite bold.
Another study mention the use of raw and refined olive oil for wood protection, but mainly focus on modified lampante oil (extra virgin olive oil may have different properties).
Please if you have more information on this topic, do not hesitate to let me know in the comment, and I may improve my answer accordingly.
: Before being outraged because I should never have put soap there. Again: I did it for years without any noticeable degradation. Please stay courteous, and support your claims with evidences. My advice against using too much soap is just a guess and should be seen mainly as a precautionary principle since soap tends to be aggressive towards organic materials.
: Oil type and cross-linking influence growth of Aureobasidium melanogenum on vegetable oils as a single carbon source, Peeters et al. (2018) DOI: 10.1002/mbo3.605
: Utilisation of chemically modified lampante oil for wood protection, Schwarzkopf et al. (2018) DOI: 10.1007/s00107-018-1336-6