I highly recommend Flitz polish. It's a cleaner applied by hand that comes in a blue paste or a thick, green liquid (not sure of the difference... but both work great) and it will remove stains and oxidation from any wheel, whether painted, polished, or chromed. It can be kind of hard to find though; I had to go to a local Toys For Trucks dealer (they sell truck accessories) and get it. Otherwise, check the net for retailers. I swear by it!