Trying to find "The One" 15w40 oil to use across my whole fleet (pre-emission diesel and gas engines)

Pick a dual rated diesel/gas oil you want to run in your 7.3L Powerstroke and use it for everything.
The old Kohler gas engines don't care if it's non-detergent/low ash or not. Oil today isn't the crappy oil of the 1950's/1960's.
The only thing that would be picky is if you have a 2-stroke diesel engine. Then you'd need a specific oil.
I tend to over think stuff as well.

For your needs, you will not see any appreciable difference. Non-emissions diesels and ULSD are really bomb-proof. I have several as well. A T444E in an International truck, and several Yanmar 3 cyl IDI engines.

For your old gas engines, any HDEO that doesn't thin out to water with the expected oil temps will be fantastic.

I'm in agreement that a non-detergent oil should really never be used, there is no advantage. I have a 1954 Farmall Super M and it get Rotella T4, and must have a million hours on way to tell. Just keeps running.
Any 15w40 oil will work. All have additives that meet API specs at a minimum. Buy what is on sale. Pay attention to the deals that you will find on BITOG. I have around 40 gallons of Delvac 1300 15w40 that I picked up for an average of $6 dollars a gallon thanks to BITOG. 5, 2.5, and 1 gal containers. 15w40 oil works in pretty much all applications from air cooled to liquid cooled engines regardless of cubic inches. What matters is that the engine has oil in it, not the brand. Don't overthink it because we all have done the same.
  • Like
Reactions: Y_K