I bought some Back to Black after reading about it here. I put some on the black plastic of my new E and it made a heck of a difference. That was about 9,000 miles ago and it still looks good. I see no reason why it wouldn't work just as good on the gray panels. You can get it at Walmart.
On second thought it might not look good on the gray. It will make them shine. Not sure if you want the gray panels shiny or not.