Most of those vessel burn a product called bunker C. Bunker C is the left over molasses from the refining process. Lots of sulphur & heavy metals. Some ships actually burn unrefined oilsands crude. The oilsands crude is environmentally a better product.

Most of the spills (volume & number of incidents) are not oil tankers but other types of ships.

No one talks about the massive natural crude seeps along the California coast that have been 
polluting the Pacific ocean for centuries.