I'm not JnJ, but I'll speculate.
Diesel oils, in general, have a higher level of anti-wear additives like zinc and phosphorous. That's why so many are switching to Rotella. Amsoil's 5W40 is actually on the thin of being a 40 weight, while the diesel oil has a thicker viscosity when hot. The diesel oil also has a higher High Temp High Shear.
Compare viscosity at 100C and HTHS:
Amsoil Diesel 5W40
Amsoil Euro 5W40