This subject comes up again and again. Search function on the forum doesn't work well, though.
1) 303 protectant. You are going to re-do this every few months, and in 12-14 years will spend close to what a new rubber would cost, which rubber you are going to replace or re-coat with liquid rubber after 14-15 years anyway.
or
2) Do nothing. Wash it with dish detergent when you feel like. After 10-12 years you will have to replace the rubber or re-coat it with a Liquid Rubber.
In either scenario there is a strong possibility that the rubber will have to be replaced fully or partially before 10 years due to mechanical damage, ex. it separated at some spot and highway winds ripped the flap further off. Time and money spent on 303 won't help then, obviously :)