What does density have to do with oil spills?

Oil varies considerably in density. Some oil is so thick that it sinks in water. However, it is very uncommon for an oil well to produce very heavy oil.

A light oil will have higher rates of evaporation, which means that some of the oil will be lost due to evaporation when it is on the surface. For evaporation, the density or really the composition of the oil is an important factor. Water temperature, air temperature, hours of sunlight, and wave action are additional factors. Loss by evaporation was considerably higher in the BP oil spill than in the Alaskan oil spill (Exxon-Valdez spill). Note that the Alaskan spill, the oil was low density.

BP used controlled burns to destroy some of the oil. If the oil was heavier and there was more wave action, this might not have been possible.