RE: Why castor oil?
the lack of suitable diesel fuels is a demand issue. Demand drives refinery configuration and plant management. Tax is a state issue. Btw, sulphur is an issue to european diesels fitted with emissions control devices. Often older tech engines are fitted to US marketed cars in order to endure reliable operation. With more widespread availability of the ULSDiesel, will come the latest tech euro diesel engines (or as you call them, high speed diesels).Low cetane is a different problem but equally restrictive due to the extensive engine management remapping required.