Vintage has given some excellent advice. Kerosene is a safer product to use than rubbing alcohol though. The kerosene won't harm the paint. I have a 15-91 that I cleaned up, it's a wonderful machine!
One way to tell if hardened oil is your problem is if you see a dark brown coating on parts that should be silver steel. If the previous owner used 3-in-one oil then the oil will harden and turn sticky. Only use sewing machine oil, it's made to evaporate, not stay there forever.
Last edited by Christine-; 09-16-2012 at 12:29 PM.