None. There is no best 5w40 for a 6 leaker.
As a former 6.ohno owner, you have to really love the things, or you should just walk away Renee.
All 5w40's will shear to a 30 weight in the engine. Nature of the beast. Run any of the 10w30 offerings, and drive.
Start putting money away, you will need head gaskets, and studs if they are not done.
If they are, continue to put money away for the disposable oil cooler, up pipe replacement, coolant flush, and etc.,etc, and etc.
BUT, if you are committed, ARP studs and FelPro gaskets are a great combination. A fresh factory Ford cooler, delete the EGR, and have the FICM worked by ficmrepair with the Atlas 40 tune on it, and the truck becomes pretty reliable and fun to drive.
Blue spring kit in the fuel bowl, factory Motorcraft or Racor oil filter and oil about every 5k, fuel filters every 10k, and Stanadyne fuel add every tank. The turbo will eventually coke up and the variable vanes freeze, but you'll get those cleaned when the studs are put in.
As I said, you have to love the 6.0 or it gets frustrating very quickly.
I got tired, traded for 6.7, and actually enjoy driving and towing now.