The Rotella that Denny is talking about it 15w-40 DIESEL oil. He had an OBS 7.3PSD and used the same oil in his Cubs. I used to use the extra that I had for my 6.0PSD. I bought a case of Rotella Straight 30 weight at a store that was going out of business, and have been using that in my Cubs. I quit using the diesel oil when I switched to 5w-40 Full Synthetic for my 6.0.
SCOTT - I called Shell's ANSWER MAN for oil questions a couple times just to ask how much ZDDP Rotella had. He said all the different viscosities had some, but depended on it's rating, but the 15W-40 I used in my 7.3L was 1400 PPM. Guy I talked to one time said the straight weights, 30W, 40W, think there was a 50W mostly to use in Detroit Diesel engines, the screaming 2-strokes, not the big 4 stroke Cummins Clones.
SON is running 10W-30 Rotella T-5 semi-synthetic in my old 7.3L now, has run 2-3 changes of it. It was his daily driver 7-8 miles to/from work for about 6-8 months. The semi-synthetic lighter weight oil made it start unassisted after work much easier. I ran a couple changes of 10W-30 every winter for the same reason when it was my daily driver. It really helped starting. I also got my best fuel MPG with 10W-30 oil on those nice spring days too.
I have not talked to SHELL since ROTELLA changed to their T-4, T-5, & T-6 formulas. I suspect it still has ZDDP, it's rated CK-4, CJ-4, CI-4 PLUS, CI-4, and CH-4. The old pre T-4 stuff was rated the same except the CK-4 didn't exist yet, and it also has an SM spark engine rating. I'm switching my old flat tappet gasoline engines all over to Rotella 15W-40 T-4. The valve springs have REALLY low pressures, no need for much ZDDP, I always thought it was funny, I started using Rotella for my 7.3L PSD, and it was the ONLY engine I used it in that didn't need ZDDP, it had those funky ball mounted rocker arms, and some sort of roller cam followers, I can't remember if it had roller tipped rocker arms or not, might have.
Far as what works in my old FARMALL M & Super H, some people still put Non-Detergent oil like the 1939 owners manual recommends.
I used Mobil 1 in my 2006 Volvo S40, using Mobil 1 in my 2018 RAM 1500 5.7L Hemi. Since I'm only driving +/- 3000 miles a year on the RAM I change the oil once a year. Short trips and stop & go driving are harder on oil than long highway trips. Seems like every trip I make is 5 miles each way.