Has anyone used the BMW Impregnator Liquid on their convertible top. I know, I know. No jokes about what it's called. It is the weatherproofing agent . The car care booklet that came with my new 330CIC says to use it every 3 to 5 times the car is washed. It seems totaly crazy to me. The stuff costs $33.00 at the dealer, and as a new owner I wash the thing at least once a week.