It is hard to separate fact from slick marketing when buying aftermarket components to improve fuel mileage and performance. IMO I notice that the graphs they display your biggest gains are in the higher rpm ranges that my truck rarely sees. Years ago Diesel Word did a few articles on the 6.0. They claimed improvements with things like, changing out all oil type fluids to synthetic. Replacing the intake and exhaust for better flow. Up sizing the tires to lower the trucks axle ratio. Installing a tuner for better performance. Drive the trucks at the lower end of the power band when possible, 1800 to
2500 when not towing. After all this expense it is going to be a long time to see any real payback unless you drive allot of miles every year. Just my opinion. Perhaps those that have tried some of these will give their opinions.