Keep in mind that diesel oils need not be (and thus almost never are) friction-modified like gasoline oils are. That means, even if everything else is the same, the non-diesel oil will be 'slicker' so to speak. One might possibly notice this in how free-revving the engine is, or in slightly better mpg. Again, this is with everything else being the same (viscosity, bases, protection levels, etc.). So, if you can get the viscosity you want in a non-diesel oil, you get other benefits.
Amsoil is tough to outguess in terms of their metallic additive levels based on their recommended applications. Since they don't seek API approval, they don't have to use the newer-technology additives that are replacing organo-metallics. Thus, either, both, or neither of the oils in question may have high levels of the old-tech metallic adds like zinc and phos, or they may instead use the newer, higher-tech adds you can't see in a UOA. You just don't know unless you buy them and analyze them.
I'd really like to hear why JnJ would switch as well. ?