I have used automotive belt dressings on fan belts in my vehicles before, and they are meant to retain or restore plyability like you are describing, and increase grip (reduce slip) for the V-belt types. Some are spray-on aerosol, some come in a tube like a grease. If you read they lable I believe they say they are recommended for all belts, but don't know if Buell (or HD-type or Victory) recommends these products for use on their drive belt.