I would try dish soap and water first, least likely to attack the finish. If that doesn't work go for something a little stronger like maybe mineral spirits. But, test in an inconspicuous spot first to make sure it isn't going to damage the finish.
Does it appear to be on the surface or has it penetrated the finish? If it has penetrated or damaged the surface, you may have to redo the finish.
As a last resort you could use something like Formby's Furniture Refinisher. Wipe it on and it desolves the old finish. Wipe it dry and apply a wood conditioner/polish like Howard's Feed-N-Wax. You can find these at home stores or hardware stores. We have 30+ yr. old oak cabinets that were showing wear, where we opened and closed the doors and a little checking here and there. We used this combination about 5 years ago and it worked great. The finish still looks like it did after we refinished and it didn't take long at all to do.