[D13]

I have found once the cam is broken in a stockish inline doesn't care what oil is in it. I ran 10w30 for 300,000 miles in the old 250, then 20W50 for the last 100, 000 to slow the monster oil leak and compensate for bearing wear. The big key is regular changes, these old motors have a lot of blowby and the oil gets dirty.
I have never had an oil related failure in any of my family's vehicles, and they got a lot of miles and cheap oil. I've always run Valvoline if I could afford it, I never run Mobil. the waxing issues with Penzoil and Quaker State were real in the 70's, but the eastern oil fields are not a significant source any more.