Pure lemon oil is very safe for ebony and rosewood fretboards, and cleans the fretboard as well as conditions it, but like everything else,use with discretion, local climate will determine how often to use it, if the fretboard looks and feels dry, it can probaly use a little conditioning. Naptha (lighter fluid)is a fairly safe solvent and degreaser, I have used it on nitrocellulose, acrylic and alcohol based laquer finishes, most of the guitars made today use catylized laquers, polyurethane or epoxy finishes and are impervious to just about everything except sandpaper and belt buckles, the only thing I wouldn't use it on are shellac or varnish based finishes, like on violins or some high dollar classicals, a couple of drops on a clean soft cloth, let most of it evaporate, you don't want to wet down the surface with it, you don't really use much more than the fumes. Cleaning guitars may seem like a simple subject, but because of different finishes and the conditon of the finish, it can get rather involved.