Bretthauer Road Oil is the responsible company. Their employee called me back and said the material left behind is "road oil" which isn't really oil at all. It is a product called "Lignin" that is a by-product of what they produce at paper mills. I asked the employee if there was an MSDS for the material and he didn't know. In fact, he didn't know much about it at all, including how to even spell the products name. I offered to send him the pictures but he wasn't interested.
One thing is for sure. This material kills plant life (ever see a weed growing out of a road that has been "oiled"?). It can't be good.