All roads have an oily sheen on them after heavy rain fall, and all cars that travel on roads and particularly city roads and highways, will have an oily coating on them. This sort of pollution has been around long before the oil spill, and will continue to be around while people use petrol and diesel in their cars and trucks. It washes off with detergent, it will not wash off with plain water.
I only have to drive around the city for a few hours to notice this film on my own car. I have also noticed oily water running off into storm drains, from roads, when it rains.
I would ask anyone to go check out their own cars, particularly if they live in or around a large town or city, and also have a look at the water going into storm drains after a downpour, (this is very obvious after a dry spell).
Information like this can be usefull, but needs to be examined properly before people let fear take over.