Originally Posted by Bruce R.
I have the first set of Porterfield R4S pads that were available for the SLK 32 ( I had to send a tracing to Porterfield so they could match them up), and I'm still running them... 
I get the occasional squeak, but never had a squeal from them. For track use you might want to consider the R4 pads, they require a little heat in them to start working properly, and are NOT recommended for the street. The R4S is a street version of the track only pads.
Well then I should thank you for the prototyping. I run the R4 pads. As you may know I upgraded my brakes to the SRT6 / SLK32 hardware last year.