Michael, I don't think much oil is left. When I pull the PTO cover, clean the magnet and swab out the bottom of the transmission, there is not much left. I drive the rear on 6" blocks. That tips the transmission down in the front (drain bolt) and tips the rear of the transmission up (fill plug).
When I first installed the Cummins in the Ford ('90) and bought the '91 Dodge, I used Rotella exclusively, oil was cheap then. Then about 10 years ago, oil doubled and tripled in price so I started using what was on sale. We have a new Walmart here so I have been buying my gasser oil there, (Supper Tech) We also got a new Tractor Supply in the next town over so I have switched to Travelers for my diesels. They have some good sales, currently $7 a gallon by the bucket. I don't think I have mentioned Rotella in ages, Lol. I do have several gallons of mixed 15/40 still on the shelf I need to use, hence the Napa. Some Delo and Car Quest left, Napa is gone now.
I don't really see any difference in the 15/40's. Several years ago I bought a 5 gal bucket of Valvoline 50w. synthetic manual transmission fluid and tried it. It is still in my '91 and Ford and they like it. My '01 did not, it shifted hard when it was cold, so on the next change I went back to 15/40.