a very long time ago, I recall hearing of a test done with NYC taxicabs. They actually tore down some engines and measured wear. They let some go without oil or filter changes for a very long time. Just topped-off. (sry, don't recall specifics) - pretty much, if thye had oil in them they were fine. (not really an apples to apples comparison I guess as they idle a lot and are parked indoors.....still....?)
I have run 5w-40 primarily because, a car can sit around in a parking lot for 8 hours in august here. Underhood temps can get quite high. I just felt that I didn't want some thin oil running out of the bearing surfaces . 'They' say there is significant wear at the moment of starting a car because you have zero oil pressure for an instant.