Bill-
I don't have any idea how the upcoming reduction of sulphur will affect fuel mileage. There many be little if any diffence in mileage.
Diesel has a rating number similar to gasoline's octane number and it is called cetane. The cetane is an average of all the components in diesel including sulphur. My understanding is the advantage for the sulphur is increased lubricity in the engine components.
Diesel fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; it has a short ignition delay period. A fuel with a low cetane number resists auto-ignition and has a longer ignition delay period. Diesel engines run just fine with a CN between 45 to 50. There is no performance or emission advantage when the CN is raised past 50; after this point, the fuel's performance hits a plateau.
Diesel at the pump can be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. Premium diesel has additives to improve CN and lubricity, detergents to clean the fuel injectors and minimize carbon deposits, and water dispersant's. These numbers are not shown on the pumps, however some pumps will be labeled as #1 diesel.
Theory would make one think that if a higher octane number makes a gasoline engine more powerful, then why not do the same to diesel. One way to boost the CN would be to add a gallon of gasoline to your tank. I don't recommend this, it will not make your engine fail or stop running, but as noted above with a CN number above 50 it does not improve performance.
I have bought at Wal-Mart a cetane booster. It does not say on the bottle what is in it but it does smell similar to toluene which has an octane rating of about 200. I could not see any improvement in power or mileage on my truck.
Hope this answers your question. When all is said and done, the refiners try to make the best available fuel to run in your vehicle, stay within the parameters of the requirements of the engine manufacturers and the federal government restrictions.
Charlie
2001 F-250 Lariat SC 4x2 LB 7.3L PSD
BD Power ECM 80 HP Performance Chip
BD Power X-Monitor LED Digital Gauges
Firestone Ride-Rite Air Bags
2005 3295RK