As a woodworker who has worked with antiques for decades, as well as used fresh rosewoods and ebonies in various pieces...
We're all getting ripped off as musicians with many products. Save your money. Most of the products sold as fretboard maintenance are marked up 100's of times over as snake oil.
Facts:
- wood used in musical instruments (or factory furniture in the last 80-100 years) is already dead and dried to normal conditioned space equilibirum. In your home or studio, it will gain or lose only a tiny amount of moisture seasonally.- Rosewoods and ebonies have natural oils, a main reason why they're not coated with finish on instruments
- Most consumer wood care products, like "lemon oils", are simply scented and/or thinned mineral or boiled linseed oil, sometimes with a drop or two of coloring.
Your fingerboard (or your furniture) doesn't need "feeding", moistening, or other additives unless it's abused, like left in a 120F attic for years. It's a dead set of fibers similar to tiny straws. If it is abused, it may not be recoverable.
Oils can clean simply by moistening the dirt and grime. Alcohols will clean, but will also remove the natural oils already present in the wood. Woodworkers use denatured alcohol to prep rosewoods, ebonies, and other exotics for gluing during builds. The natural oils in some woods prevent glues from adhering, so it needs to be cleaned before assembly.
Antiques sometimes moved a lot because they were made from wood that wasn't dried to the standards of the last 100-120 years.
A LIGHT wipe, as in you barely know you applied it, with drugstore mineral oil or home center boiled linseed oil can remove finger gunk and caked dust. Too much of a polymerizing oil like BLO, will actually build to a finish. For example, TruOil and Watco Danish Oil are mostly BLO, you build a finish via many applications. Too much of a non-polymerizing oil, like mineral oil or "lemon oil" will stay sticky for a long time and trash your strings.
A great data point are studios or rental agencies that own many instruments. Most simply keep them relatively in tune over decades, in normal conditioned spaces. Nobody is "conditioning" or "feeding" the fingerboards.
Your best friend may be a simple very slightly dampened microfiber cloth.