There are always going to be incredible "stories" of magical increases out there but I don't think any oil maker would try to say that thier oil would give you a 35% increase in FE. As for Amsoil, I used to race for Mazda and Amsoil performed better than anything else we tried. Lower coolant temps, lower tranny temps and better power. Of course we didn't worry much about FE even during 24 hour events. UOAs can vary between different labs on the same samples...but I have never been let down with Amsoil. I also like thier Ea series of Air and Oil Filters.
I ran a few back to back UOAs with Series 3000 5W-30 and then AME 15W-40 in my little Diesel and the AME 15W-40 returned better wear numbers and TBN scores. You have to see what your vehicle runs better on and then stick with it. Now I go ~10,000 miles on the oil and filter with no problems which means it is a bit less than three normal 3,000 Dino Oil Changes...plus the AME is for Marine and Diesel applications and suspends soot better than conventional oils.
Whether it helps FE much...too small a variable to properly test in the real world...but I would bet that my engine will last longer and I waste less oil with synthetics.
2006 Jeep Liberty CRD...Founder of L.O.S.T.
OME 2.25" Lift w/ Toyo Open Country HTs 235/75/16s
ASFIR Alum Eng/Tranny/Transfercase/Fuel Skids
2002 Air Box Mod...Air Tabs (5) on Roof...(3)each behind rear windows
Partial Grill Block with Custom Air Scoop and 3" Open Catback Exhaust
Lambretta UNO150cc 4 Stroke Scooter