PO's of my engine used a shop near the man's business for their service that had Chevron 10W-30 bulk dino. The wife drove the Mustang up and down I95. Although I still give it a D grade for the coked rings and bottom end, I will say that the Chevron bulk 10W-30 only allowed a very light coating of varnish on the top end. In fact I couldn't really see it on my inspection at purchase. I think they stuck to a good maintenance schedule except for the air filter near the end of their ownership. It's a very nice car.
Hirev: I will measure the bores and ring gaps when I take this engine apart in a couple of years from now. But for now I can influence both the oil consumption and fuel mileage with oil viscosity, so I know the rings and/or bore have been worn.
Bill in Utah: I wish I had your air and driving conditions for my engines. Conditions here are brutal, and my driving doesn't help any. I have never seen conventional oil take care of an engine I have owned or bought. I switched to stiff Mobil 1 blends and full-synthetics long ago. These oils work for the long run and are worth the money.
The magic engine-curing oil is just M1 mixed with some HDEO. I couldn't believe the amount of carbon and hunks-o'lacquer released into the drain pan. That has stopped and my oil consumption is 1qt in 1700miles with 5W-20, and about 1qt in 2500miles with medium 30weight oils. I should polish off my cleaning effort with an ARX treatment (I didn't know it existed until recently).
I hope I didn't offend anyone with the revelation of my experiences. But they are what they are. BTW I also give M1 10W-30 a D grade (BAD) for noise, harshness, and leaking. It's the oil that gives all synthetics a bad name, and they need to fix it SOON! It gets a D despite the acceptable wear and good engine cleanliness. I have used many, many gallons of it as a blend oil, but by itself in the crankcase it makes me nothing less than furious.