It may just be crap in your fuel tank. When I was a kid, my dad worked in the oil industry. When I would "help" (tag along from sheer boredom) him, I can recall pumping out underground fuel tanks. It still to this day amazes me just how much water, and other debries can actuallt lie in these tanks. I mentioned it to him the other day, and he said to me, the only time there's any danger of getting sh1t from a servo, is when they are filling their tanks.
So, NEVER FILL YOR CAR WHEN THE TANKER IS THERE!!!!!!!!!!!

Thats my advise to everyone.