If the vehicle that drops the oil is slow moving or stationary and the ground is level, the oil will diffuse into an almost perfect ring. As the lighter elements separate and flow outward these will take on the appearance of a bulls eye, flower or ring nebula (supernova remnant in real life).
Cometary and Meteor Shapes
Similar to a ring, but the ground is sloped or cambered. As the oil separates, it flows downhill with the lighter elements moving faster on a thin film of water. This creates a "tail" that resembles a comet or meteor. In addition to the example shown here, the image at the top right of this page is a prime example.