Start with mild solutions and work up from there. Remove the pad from the stock or be careful to keep cleaners off the wood. In order of aggressiveness, I'd try Simple Green, dishwashing liquid, WD-40, mineral spirits, lacquer thinner. Use a toothbrush on the face if textured, and ScotchBrite on the smooth parts. When you get up to the lacquer thinner, you will probably be cleaning by removing minute amounts of the rubber, carrying the crud with it. Be careful. You could also step up to acetone or MEK, but you run the risk of damaging an old, dry pad. The lacquer thinner may have the advantage of softening the pad slightly.