Before you begin to clean a wood finish, it's important to know what finish you have. I like mild cleaners like Murphy's Oil Soap for light general cleaning. Lemon oil is a little stronger, and the Endust mentioned above is a citrus cleaner. Rubbing alcohol or denatured alcohol won't hurt many finishes, but will totally remove a shellac finish. I really like using an ammonia solution on seriously grungy stocks that are heavily oiled because ammonia turns gun oil and skin oils into a water soluble soap. This must be thoroughly rinsed off of the wood, and I only use it on stocks that need de-oiling and a refinish. Mineral spirits and turpentine is a fairly mild solvent that won't harm varnishes, but they will begin to remove some oil finishes if left on the surface long enough.
Abrasives such as pumice or rottenstone have their place, and used with BLO or another type of cleaner, will certainly cut heavier grunge. But they will also cut the surface of the finish. Depending upon how coarse the abrasive is, it may dull the finish. Anyone who doubts that Scotchbrite is very abrasive should try using it to clean bug guts off of a windshield. The stuff will scratch glass. Don't ask me how I know.
