Checking out what FCC was on the wikipedia lead to some enlightening reading....

One way of phrasing what Im getting at is:
When the gasoline is in the hummers tank ready to be consumed; Is there already a larger CO2 price tag on the gasoline derived from Heavy than on gasoline from light sweet.

Heavies have more carbon and less hydrogen than lights, so more CO2.  It's not a gigantic difference, though.
And wikipedia is a good source if you want a quick look at the unit operations in an oil refinery.