I have not seen UOA that "fleet" customers were getting that the TDR article referenced when the CJ oils came out. I think this would be very interesting info as to what was causing reduced miles out of the oil. What I do know from our own experience is there IS a reason they reformulated the oils after they came out.
When oil fails it fails quickly and damage is also quick, expensive to repair, and may not get you home. Having scuffed a piston, seen "Low Oil Pressure stop engine" alarms on a extreme hot day towing, viscosity jump to nearly a 60 weight, extreme soot... I get phone calls from the oil lab.
Those with a daily driver that don't tow, ever, can just skip the rest of my rant. :-laf
Towing a 24-28' cargo trailer around the 121 degree summers in Arizona stuffed full of auto parts and tires. Add to this mountain grades that go on for miles. Mountain Grades starting at 7% up to 18%. Literally uphill both ways on a delivery route we had.
I should run a synthetic oil. But changing it every week would be expensive...

In one case it could have saved me from scuffing a piston when the cooling system wasn't doing so good. The engine nearly locked up on one grade running hot and then continued running. 7000 miles after cooling system repairs with UOA having lots of things in red the engine finished burning through a cracked piston in a scuffed cylinder and blew oil everywhere from the crankcase pressure. Oil changes were every 3000 miles, once a week, and the UOA was miserable, cooling issue engine aside, even though the three 6.5's I ran vaporized a quart every 500 miles. So I really changed the oil 2x over a week by adding a quart every day. Yes, older 6.5TD working hard on modern emissions oil. Yes, the oil spec for the 6.2 engine was originally straight 30 weight oil.
Prior to this we used a 2008 Duramax that the auto liked to lug 6th gear (double overdrive) and heat things up. Ever boil a battery dry from underhood temps? Even with a 2 quart aftermarket oil cooler: after kicking off the trailer at the end of the day on a hot afternoon with the engine idling then a shift to drive would trigger the low oil pressure alarm for a moment. Oil pressure below 9 PSI. GM has likely "buffered" this low oil pressure spike since. Tends to happen in extreme hot weather near the end of the oil's 9k mile life. The alarm, of course, means the oil is done. Extending the oil drain intervals to me that I read about others doing would be a bad joke for these conditions.
My 2nd 6.5TD I hot rodded a lot and with the BD spool valve noticed it kept more heat in the engine from the exhaust. Not being afraid of 1550 EGT on that engine it held together just fine while we perfected the tune to run a little cooler. Later during the hot summer with 1400 or less EGT and 210 or less ECT I got a UOA back saying I more or less ruined the oil via overheating it (and some soot) as it was nearly a 60 wt. Was a 15w-40... I switched to synthetic on that engine. So much for the extra soot holding limits of the oil.
In conclusion you can reach the limits of conventional and synthetic engine oil. The EPA mandated changes to oil make that limit easier to reach. Just a slightly higher setting on the programmer maybe gets you there.
IMO if you want to really see the changes you have to "test until failure" rather than using just UOA. UOA just tells you you didn't reach or are near the limits. Scuffing from oil failure, heat failure, soot loading viscosity changes, soot build up, overloading via lugging and resulting metal to metal contact, and fuel dilution limits can show you the real changes to what you once thought was a good oil. Sure the new "neutered" oil will work in a modern engine. The older engines are out of warranty so it must be miles and age killing the camshafts rather than oil that is no longer designed for the "old" engine. The EPA is quite happy when the old engine fails and has to be replaced with a new low emission engine. Yeah, we have seen changes "harm" old engines in the past and the emissions Diesel oil has affected oil drain intervals for the worse on older engines.
I am curious if the reformulations have corrected oil drain intervals for fleet operators lately.