dazmond is right, it is very easy to scratch the sills when cleaning BUT you can just as easily train yourself not to do so.
the gardiner sill brush definitely helps, but if you are scratching the sills you will hear the brush stock hit the window, and you can hear it scrape along the sil also.
once you are aware of the problem it is easy enough to eliminate from your cleaning technique. just stop short of letting the brush stock hit the window.
and yes you can clean the dirt out of the scratches with a PVC cleaner.
hope this helps