I have the s100 version tom. So far ive used it on wheels wells, lower body panels, and around my exhaust openings where it gets sooty. It works great, it pretty much dissolves everything, and its very safe, best of both worlds kind of. Im willing to bet the s100 version like I have is 100% the same as the P21S at a few dollars cheaper.
I spray it on, let it dwell for a few minutes, and spray it off with a strong stream from the hose. Probably good to clean engine bays too. This might also be a good safe wheel cleaner to use, Im gonna try that out soon. Something that is probably safe enough to use all the time, but will most likely clean pretty well. And at 10.99/33oz S100 refill, its not too expensive.
O and I havent tried it diluted yet.