A while ago someone posted a DIY and i tried it on my 1995 318is. I used castrol super clean. The other materials I used were paper towles, latex gloves, a wheel brush, a kitchen sponge, and a rag. I spraid it on and let it soak for about five minutes. I did this in about two foot pices at a time. Each side took me about 30 min and i have no grey left...none. It looks brand new. Then i got my front and rear bumpers painted (it cost me $300).
My only advice is to...
-wear the gloves
-do it in a parking lot or a street..it will ruin your driveway
-I used i pice of cardboard to block the over spray from geting on my paint
-Scurb hard..once you get through the top layer it gets easier
-Be carful this stuff is very lethal
-Once the grey turns really liquidy then wip it off with a rag, then start scrubing agian
-Once your done you'll look like this
P.S. its worth it