I was just wondering about this the other day.
More specifically, how do they remove the sulfur for ultra-low sulfur diesel. Diesel used to be cheaper than gasoline, but now is more expensive.
Gasoline also has low sulfur requirements.
Hydrodesulfurization
http://en.wikipedia.org/wiki/Hydrodesulfurization
There is siginficant competition in the different processes used to remove the sulfur.
http://www.fwc.com/GlobalEC/Refining/sulfurtechnology.cfm
http://www.cbi.com/technologies-services/hydrodesulfurization/
Leaded gas required an additional process and product as opposed to unleaded, yet, leaded gas was cheaper.
I eventually thought the reason was to clean out all the leaded gas as quickly as possible so the tanks would be re-filled with unleaded ...
and as Kurt Vonnegut Jr. so aptly put it;
and so it goes.
Right after I bought my first diesel pickup, Chevy 2500 Duramax.