Most All Purpose Cleaners, agitated with a stiff brush, should do the trick.
FWIW, we often turn to Malco's Leather and Plastic Cleaner (Dilute 1:1).
Also, we like Automagic's #58 Leather Conditioner with Lanolin. Apply it with your bare hands and allow to set. Seldom does it require removal of residue.
Good Luck!
Jim