Refined motor oil has a limited number of constituents, much of the original material has been removed for other uses.

The much more homogeneous refined product would tend to spread much more evenly than most crude oils. Gaseous hydrocarbons, heavy hydrocarbons, and other constituents like sulfur compounds have been removed. Refined oils often have added constituents to make them flow more easily, and to help them keep their lubricating properties at elevated temperatures.

Note the clumpy mess on the beaches, which is the heavier part of the crude- the gas is widely dispersed in the water, and the lighter, more fluid elements are partly separated from the heavy when diluted in water.
Crude oils are often rich in sulfur (as hydrogen sulfide, etc.), which are not only extremely toxic to life but chemically combine with other elements in the water. Much commercial sulfur is a byproduct of oil production, recovered in the refining process.

I don't know the composition of the particular crude that is gushing into the Gulf, only that it is rich in gaseous hydrocarbons. I suspect that it is rich in sulfur but I don't know if that is correct. If anyone runs across its specific composition in some article, please post.

The gusher is in the northern Gulf, the Gulf to the south (Mexican control) is likely to be little affected since currents tend to take northern Gulf waters toward Florida and out into the Gulf Stream. If the crude oil is sufficient to cause warming in the northern Gulf, the southern part would remain cooler. Dispersion of the crude will be uneven, so effects will vary in the northern Gulf.