From a science blog. I don't know if I can post the site without getting banned. I suggest you copy some of the text, post it in google, then find the whole report.

....

" As for crude oils, a very decent analysis by the American Petroleum Institute shows that all are toxic, but their effects vary with thickness and with the different chemistry seen in say, oil from the Gulf of Mexico and oil from Kuwait. The best estimate I've seen for South Louisiana Crude - after hours of exasperated research - comes from thesis work done at Louisiana State University several years ago. For instance, the study found that Louisiana crude had an LC50 of 4250 ppm for the warm-water loving killifish.

This suggests that crude oil is less acutely poisonous than chemical dispersants. But here's the really interesting finding in that terrific little study. Adding a dispersant - specifically Corexit 9500 - made the oil more poisonous. A lot more poisonous.

The "dispersed" oil had an LC50 of 317.7 ppm, making it more than 11 times more lethal in its effects. The study found a similar worsening for white shrimp, although not quite as dramatic. "Dispersed oils were more toxic than crude oils," noted the report.

Oh, definitely. Still, you might argue that this is only a master's thesis conclusion. But as it turns out there are plenty of other studies raising very similar warnings and they go back quite a ways. A report in the journal Environmental Toxicology a decade ago concluded that "LC50 values indicate that dispersed oil combinations were significantly more toxic to these organisms than .. crude oil." Another study, this time of snails and amphipods reached exactly the same conclusion. ...."