Here’s what I can advise about fingerboard oil. (Spoiler alert: there is no indisputably “best” fingerboard oil!)
What’s the point of fingerboard oil? Given, it’s to coat the fingerboard surface to slow or impede moisture loss or gain. This is desirable because it slows the fingerboard from shrinking or swelling on the guitar. And since we usually chose hard, glassy woods such as ebony as ideal for fingerboards—ebony being notoriously reactive to ambient humidity changes—sealing is important. Literally anything oily: motor oil, Crisco, 3-in-1, lard, will do the job. But they stink. So there are additional criteria.
If sealing was the sole criteria, lacquering the fingerboard surface would be ideal. Lacquer wouldn’t display the problems that oiling presents, as I detail below. But it makes little sense to lacquer a surface meant to be abraded constantly by the fingers and by the strings. Experience shows that lacquered fingerboard surfaces becomes flaked and pitted within a year or two of hard use. So, although many Latin-American luthiers lacquer their fingerboards on their folk instruments, it’s generally considered bad practice. You probably have seen what happens to lacquered maple fingerboards on old Tellies and Strats. Coatings become easily uncoated. The only good thing going for lacquer is that it doesn’t attract and hold dust and crud, like the other alternatives do.
So what’s left? Oil or wax. First, wax. My finishing-chemist friend says that a light coat of any clear furniture wax, like Carnauba, would be ideal. He says that you’ll need to replenish it periodically as it wears off, but new wax actually redissolves the old, and if applied moderately, won’t build up: it’ll always have just one coat of wax, after buffing. And after it’s buffed and dry—and this is a good thing—like lacquer, it doesn’t attract and hold any dust or dirt. So wax has no down side, it seems—just that I haven’t heard of many luthiers waxing their fingerboards, although I’ve heard that some old timers used wax. But I’m not in the habit of using it, myself.
Oil, which is considered a penetrating finish, is far more commonly used than wax. Besides sealing the surface—albeit it, temporarily—the advantage of oil is that it does not flake off or get pitted. But on very hard, tightly-pored fingerboard woods such as rosewood or ebony, very little actually penetrates, so it eventually wears off and needs to be replaced at intervals. This is particularly true in areas of the fingerboard that are played on. But on areas that are not often played on, the stuff just sits there and builds up.
As far as oils for wood finishing is concerned, here is what my finishing-chemist friend says: there are basically two kinds of oil: drying and non-drying. The non-drying don’t undergo a chemical reaction (i.e., oxidation). The most common non-drying oils are lemon oil, orange oil and mineral oil. These oils have a very slow evaporation rate and remain on the surface in a microscopically thin semi liquid state for days – even weeks – after application. So they’ll wear away where the surface gets used a lot and build up and attract dust and dirt where you don’t play. But many guitarmakers like them, and recommend that their customers periodically (i.e., on every string change) replenish the playing surface with these non-drying oils—and less often, scrub the buildup off with spirits of turpentine and follow with a new coat of their favorite non-drying oil. Some people like to do that because they think they’re coddling their beloved.
Drying oils such as tung oil or linseed oil actually undergo a chemical change as they dry and attempt to bond chemically with the surface. As they become hard through oxidation, they become difficult to remove and over time the accumulation of layers of dried oil forms a shell over the surface. So they build up and they don’t come off, looking rather badly after a long while. So I stay away from them.
Alas, there is no consensus among “experts". My teacher, Michael Gurian, clearly an expert in his field, used his own blend of Cocoanut and Eucalyptus oil. All I could get out of him to justify it was that it smelled nice and always stayed a little oily because they are non-drying, so it would actually lubricate your fingers as you played and your fingers wouldn’t squeak on the wound strings while playing. He used to supply it commercially, but he doesn’t anymore. My stash of Gurian Fingerboard Oil is long used up, sadly.
Most current commercial fingerboard oil preparations usually say they include something called “lemon oil”. You’d think that it came from lemons. In most cases, you’d be wrong. My finishing-chemist friend revealed to me that “lemon oil” on the market is artificially-lemon-scented-whatever-oil-they’re-trying-to-get-rid-of, drying or non-drying. So you take your chances. REAL lemon-oil preparations are a good candidate, and like most non-drying oils, they will temporarily seal the wood surfaces, making the wood look nice and smell nice. So you wouldn’t be WRONG to use them—assuming you can confirm that it actually comes from lemons so it’s lemon oil and not “lemon oil”, AND that they doesn’t contain any silicone oil. Before applying any oil to your fingerboard you have to know that it doesn’t include any silicones or silicone oil of any sort. “Contains no silicones” is what you’re looking for on the label. Silicones are very bad to have on your guitar. So most commercial “furniture polishes” are verboten because typically they do have silicones (i.e. Lemon Pledge and the sort). Silicones “migrate”: they spread from the fingerboard to your fingers and eventually cover the entire guitar—making any future reglueing or finish repairs problematic. My warranty is void if I find any evidence on my guitars of silicone polish or any fingerboard treatment that has the typical slick, silicone look.
Finally, after using up my last jar of Gurian Eucalyptus/cocoanut fingerboard oil, I’ve settled on a commercial preparation that is essentially light mineral oil, thinned out a bit with mineral spirits. Some commercial “lemon oil” preparations are actually lemon-scented mineral oil. Read the label. Look up the company’s website. Ask questions. Finally, Baby oil, which is essentially mineral spirits will serve as an excellent fingerboard oil. If its too thick to handle and apply, thin it out with a bit of mineral spirits.