I will just state what I have from my experience with over 2,000,000 miles of car, PU and van driving.
I have never had an engine blow or a bearing failure. I have experienced excessive engine wear on one engine. I took a 1969 Ford van with the very dependable and strong 7 main bearing block 240 CI and built it in the race car shop I worked in. We used TRW everything back then in 1974. It had TRW forged pistons, TRW rocker, push rods, springs, valves, lifters and cam. When I rebuild an engine I replace everything but the crank, rods, head and block and they were reworked.
With the crank installed and all main bearings torqued you could spin the crank with one hand as it should be. That van was driven 467,000 miles on same block, head, crank and rods. I took the engine out every 100,000 miles and rebuilt it. On the third build I decided to try Mobile one synthetic. We did break the engine in using the normal 10-30 Havoline I had been using for 300,000 miles with no issues.
I had probably 1,000 miles on the engine and took on trip across the U.S. that was I think around 6,000 miles. The engine used quarts of oil. By the time we got home the engine was ticking and not running right. When I pulled the valve cover and took out a push rod they were worn to a point almost, the ball was gone. Rockers shot lifters shot and cam shot. There were no broken rings to cause the excessive oil use.
I rebuilt again using the exact same parts but did not use the synthetic oil and was still running fine at 467,000 when involved in a crash. We had driven it that year non stop from N.C. to Montana to go hunting and non stop back pulling a trailer. I will never use synthetic oil in an older engine. I do not know if Ford has ever gone to a full synthetic in new cars it was a blend for my 2002 F-150 that has 286,000 miles on it without any engine issues.
At a little over 100,000 miles I did drain the rear grease on the F-150 and went back with full synthetic with the friction modifier for the limited slip. In just a few thousand miles the pinion bearing started to howl. I went to wreck yard and got complete rear for less than it would cost to replace the bearings. Needless to say I do not use full synthetic in the rear end either.
You know they use automatic transmission fluid in the T-5 5 speeds in the Fox bodies. Would you put that in your toploader? NO it would burn up almost immediately because it was never designed to use it. The 71 - 73 was never designed for full synthetic motor oil. It was designed for high zinc regular 10-30, 10-40 oil. The seals are not designed nor the clearances or wear surfaces with synthetic in mind. They did not have it then.
The engine in my Mach 1 sat for 37 years and when I pulled the pan and took rod cap off there was still oil in the bearings and not a scratch on the bearing. Nothing had rusted nothing stuck. Cam, cylinder walls were all fine. You would have thought the engine had ran the day before. Why would I want to change to an unknown when a know oil has never had an issue.
Most engine failures are due to abuse, over reeving, over heating, starving of oil, etc.
The only bad thing I will say about regular oil is never use a paraffin base oil like Quaker State. It will build up more sludge and deposits inside the engine due to the paraffin.
I have used regular oil in flathead V-8, 200 CI 6 cyl., 250 C.I. 6 cyl, 289 2-V, 289 4-V, 302 2-V, 390 4-V, 352 2-V, 351 2-V, 351 4-V and have never had an engine failure.
I will stick with Havoline with some zinc added for the flat tappet cam and heavy valve spring pressures in the old engines.
Look at the valve sizes and valve springs in today's Coyote engine. They look like they came out of a lawn mower compared to a 351 C or 429. They got rid of the weight of the lifters, huge valves and push rods and all that spring pressure and inefficient rockers so the oil does not have to bear 1/3 of the load it did in the past.
The seals in the new engines don't even use the same materials or designs to retain the fluids. None of my old cars will ever see synthetic oil used.
My two cents worth.