I completely understand where you're coming from on the truck; my dad has been gone for many years now, and we still have his...an un-modified, pre-emissions, Cummins-powered Ram in excellent condition. It's worth a small fortune on the used market, but it's not going anywhere!
(I should mention that I'm not compensated in any manner for my time spent here, nor receive anything for recommendations; just my personal thoughts.)
In regards to what I mentioned yesterday, SAE J300 governs viscosity specs:
The oil you're using starts out at 10.6 cSt (@100°C), and finished at 7.75. This is a huge drop of around 30% or so (quick math in my head). Usually, you'd think about fuel contamination, but that's not an overriding issue here. Through some mechanism, it's sheared-down quite considerably.
Air Filter - As I'm sure you know, this is critically important. I'm a fan of the Fram Tough Guard for its excellent construction, coated media, and nice price point. TGA3901 is the part number, and it's on the shelf at Walmart for under $10. Before changing the oil again, take the time to confirm everything in the air intake (post filter) is perfect (no leakage, clamps tight, etc.).
Oil - Mobil 1 Extended Performance is great stuff. With that said, it didn't perform well here, and its formulation isn't exactly geared towards your application. A strong 10w-30 (HTHS >3.5) would be a very good choice. Do you have a brand preference?
Oil Filter - Don't install a bypass system. Full-flow filters are achieving excellent levels of filtration these days...
https://bobistheoilguy.com/forums/ubbthreads.php/topics/4299547
XG16 would be the stock size filter. XG8A would be the oversize version. These are both Fram Ultra's, which are again available at Walmart for a very good price.