Flying J is a dichotomy. You'll generally get great fuel mileage with their gasolines, but not so much with their diesel. Flying J owns two large refineries outright (one in North Salt Lake, and they bought Shell's Bakersfield, CA refinery last year), and own majority stake in 4 (maybe 5) other refineries, as well as minority stakes in several others. They also own Longhorn Pipeline, the 700 mile-long pipeline across Texas. They make a lot of their own diesel.
At most Flying J locations its diesel has a cetane level slightly below that of most other retailers (which is why they tend to be a few cents cheaper than surrounding fuel stops). Flying J's diesel is always at least 40 cetane, and up to 45 cetane, but usually in the lower 40's. Others are usually at least 45 cetane, up to 50 cetane. (CARB diesel is 50 minimum, BTW).
I'm still gathering data, but it looks like the increased fuel mileage of TA diesel offsets the extra cost, even when you figure in the cost of a free van shower at the J. It'll take several more fuelings to gather enough data to make it mean anything. I haven't fueled up enough at Pilot, Loves or the Petro yet in order to make any kind of determination on those. But early results show nearly a 10% increase, a little more than 2 MPG, with TA fuels versus Flying J fuels. That's a lot.
But, the question will be, is buying the cheaper Flying J fuel, and then adding a cetane boost to get the same mileage as with TA fuel, the same price as buying TA fuel in the first place (and for a van driver, with the added cost of buying a shower on top of that)?
Worldwide, diesel engine makers are pushing for adoption of the WWFC (World-Wide Fuels Charter) which would have us using fuel with 55 minimum cetane and near-zero sulfur levels, which will aid in reducing cold start emission, raising fuel economy, and cut engine noise. That'll probably be expensive fuel. The higher cetane fuel also goes against the new engine that CAT is developing (the HCCI, or Ultra-low-emissions homogenous charge compression ignition) which runs with dramatically fewer emissions and greater fuel economy on 45 and lower cetane, but just the opposite at higher levels of cetane.
