Just to muddy the waters a little more, I have been using Cummins Premium Blue in my 94 since the truck was new. It is formulated to their specs by Valvoline.
If I get to the point I can't get it easily, I'll probably switch to Delo 400 and buy it from Costco.
I doubt if there is much difference between any of the name brand oils as long as they meet Diesel requirements.
When I bought the truck, the dealer promised free oil changes as long as I owned the truck.
I asked what oil they used and was told they use Quaker State 30 weight.
I declined.