Maybe none. Too many variables to state whether a commercial truck is going to see any mpg improvement. I have never seen any appreciable difference in mpg from using a full synthetic 5w40 or a synthetic blend 15w40 I use now in my truck. Cost is a major difference though, with the blend being cheaper. A tenth or two might be seen, but it would be inconclusive that the oil was the benefit. Since the driver himself is almost 30% the variable in a truck getting or losing mpg, there can be a placebo effect in play. If one does something, like oil type change, to gain mpg, they might actually drive a little better to prove the point, albeit even if they are not aware they are doing so, and the better driving is what got the mpg than the the oil type change.
For me it is about cost benefit analysis. I get fantastic oil sample results using a Schaeffer 7000 15w40 synthetic blend (25% PAO and 75% Group II+) that I can get delivered free to my door, complete with free oil sample kits, for $980.65 for a 55 drum. That is roughly $17.83 a gallon delivered free, only state sales tax. That beats most anything on Wally World's shelf, and I don't have to go get it. It is available in gallons, 5 gallon pails, 30 gal drums, etc also. SalesRep here on the site can fill you in. And this stuff has a killer amount of zinc and moly. I get the lowest wear numbers I have ever seen on any oil sample reports in over 2 decades.
