I've purchased my first L lens, a 70-200mm f/4L USM purchased used through eBay for 510.00 US. The lens is awesome, especially coming from a 18-55mm kit lens I got with the T2i. However, the rubber grips have turned a sort of gray color. I know this is purely cosmetic and doesn't effect the image quality but, I want to fix it. Is there something certain I should use on the rubber so I don't ruin the lens? Or, should I just leave it be?