[DanEC]

My theory is that it's a nautral build up of oxidation and road grime that accumulates and needs to be cleaned off periodically to get back down to clean rubber. I'm sure I'm not alone in giving my tires a quick wipe with a sponge and soapy water when washing the car and then applying more dressing over the top of what the quick swipe with a sponge left on the tire. What has always worked good for me for cleaning tires is 409and a brush, but need to be careful about getting it off of paint and wheel surfaces pretty quickly.