FWIW I've ran a few partial (1/2-3/4) tanks of B20 in my 12 CTD. Never had any issues. That was a couple years ago. IMO I don't see how filling a partially empty tank with some B20 would make a difference. I won't run it 100% of the time even with the CAT dual filter setup I have.
What I'm having a hard time wrapping my head around is what exactly does Cummins think will happen if I do run B20 100% of the time. Is it an injector, HPFP, etc problem? If I were deleted then it certainly wouldn't be an EGR-DPF problem.
I've even asked the local Ram service guy and he didn't have an answer either. Seems that no one knows for sure.