The stock 18" wheels are hard to clean since they are polished aluminum with a clear coat over top. I just use hot water, soap, and a microfiber towel to clean them. My center caps actually match my wheels pretty wheel. The only annoying part is the areas of the wheel where it look like someone took white out and put on the wheel which is from air and other contaminants getting under the clear coat.
I am not sure if the center caps are cleared coated or not since they are just plastic with a polished aluminum finish. Maybe try some rubbing compound to see if that will bring the finish back.