Personally, I use Fleetguard & Delo in the Cummins. The oil comes from Wal-Mart and the filter from Cummins. This past winter I switched to Cenex Superlube 10w-30 as it will pour at 0°F and the Delo will not. I will permanently changeover to Synthetic before winter.
I work at an OEM and we put Cenex 15w-40 oil in the Detroit & Cummins Diesels. The customers are free to use their favorite brand as long as it meets Cummins & Detroit spec's. That pretty much means any 15W-40 oil that is API CH-4 or equivalent.
We also us Cummins & Detroit CNG fueled engines and the oil for these beasts is much different than diesel fueled units. For those we have to use a extremely low ash & high phosphorous oil. For them we use Mobil Delvac GEO.
I use<em>d</em> conventional Mobil in the wife's Caravan, <strike>at least</strike> until <strike>the next change</strike><em>today</em>. I just bought Mobil 1 Synthetic for a permanent changeover. I prefer Purolator filters for the van, but will <em>no longer</em>use <strike>premium</strike><b>ANY</b> Fram<strike>, too</strike>. The premium filter is a pretty well built, for a Fram,<em> or so I thought. I cut open the filter can today. The bypass valve & screen is great, the can is heavy enough, the anti backflow valve is good, but it still has the same CARDBOARD end caps on the filter media. I wonder why Fram added a nice bypass filter screen?</em>
John