MF, that would depend on thwe wording in the manual. "Recommended" versus "Required".
In most cases, I think your okay. But the MDS will not function properly without the proper viscosity, and therefore I think is required. I think that Amsoil dealer is stretching the M&M act.
I think while they cannot indeed specify a brand, they can spec a viscosity. Like specing a friction modifier for a limited slip rear end. Or a certain weight of gear lube for the diff. Do you really think if you used 50W instead of 90W, you would, or even should be covered?
The 6.1 of course does not have MDS, but I'll still use what they recommend.