Naphtha .. i.e. Zippo lighter fluid.
It is gentle on typical wood finishes, will easily dissolve dirt and oils, and evaporates quickly so it doesn't expand the wood. It won't react with nylon, so a nylon bristled brush can be used to scrub into tight places, like the areas of the frets on a guitar, or the texturing on the grip.
Probably its overkill. A nylon brush and some soap and water would likely be just as good for your purposes.