Crude oil can be lots of different stuff. From light sweet crude, which can be almost good enough to run an engine with, to quite nasty sour crudes, with lots of sulphur containg compounds that are quite noxious. Indeed some wells contain very dangerous levels of hydrogen sulphide. So leaking crude can involve significant levels of quite nasty stuff, other than the simple oil itself.
Refined oil is mostly pretty benign. We treat out car engines very well, and we work hard to get the nasty stuff out of the fuel and lubricants we put into engines. We certainly like to remove the sulphur. Modern lubricants have all sorts of additives, so they are not just oil. Used engine oil can pick up all sorts of junk and combustion products, and is likely to be quite nasty.
Crude oil is usually a lot of different compounds. The chemistry that created the oil from the basic material really doesn’t care much what it makes, so in addition to the alkanes that you want, you get a lot of long chain, and cyclic hydrocarbons. Cyclic hydrocarbons can be toxic of themselves. Think benzene, and other aromatics. The long chain hydrocarbons have very low melting points, and are the usual constituents of tar and waxes. So when the shorter chain stuff evaporates or is consumed you are left with balls of tar to deal with. Refining crude gets us all sorts of interesting chemicals as residue left after we have got the good stuff (short alkanes mostly) used to feed our cars. It has been observed that we are lucky we have such an appetite for plastics since we would have a serious disposal problem for refining residues otherwise. Plus waxes, tar, creosote, etc are all useful.
Oil in the water takes some time to be eliminated. Whilst it is there it can damage wildlife, simple stuff like oily coatings killing birds, coating plants leaves and killing the plants, and so on. In the water the lighter oil fractions are eventually eaten by bacteria that specialise in oil. However in order to consume the oil, the bacteria need quite a bit of oxygen, so you can get large areas of oxygen depleted water, which kills all the fish in it.
Oil leaks from the ground naturally in many places in the world - indeed the Gulf of Mexico is full of such leaks. Some of the tarballs that were found on the beaches during the Deepwater Horizon disaster were not from the leak, but were from natural seeps. The problem with Deepwater Horizon was that it so utterly overwhelmed the natural processes that normally deal with the seeps. So much so that it was not clear how badly the ecosystem would be affected, or for how long. Once well enough dispersed that with enough oxygen in the water, additional nutrients and warm enough water, bacteria would eat the oil. But residual contamination of the area with tar, and dangers of hypoxia creating large areas of dead sea, with damage to the fisheries - it could have been very bad. It wasn’t as bad as all that, but it sure wasn’t good. As it is, the very slow replenishment rate of oxygen in deep water was a significant worry.
