Interesting observations, thanks for sharing.Tried 0/5W-20 for two years in the Ranger, 4Runner and F150. A few downsides and no benefits. Now we have gone the other direction and seeing what 10w-40 does.
The Ranger didn't care outside of noisy lifters. It will get the 10w-40 with this Summer's OCI.
The F-150, an already noisy 4.9l tractor engine, was very noisy and seemed to lose power pulling a trailer against strong headwinds at highway speeds. 10w-40 seems a better fit (265,000 miles on the un-touched engine).
The 4Runner (275,000 miles on the un-touched engine) doesn't seem to care except one day when the wife, her car, got in and started it for the first time after sitting overnight. The engine made what sounded like a rod knock for about 10-15 seconds before it built oil pressure. It was parked in front of the garage and I had the door open and it caught my attention. She got out of the car and said "what the heck is that". I said I don't know but I think I have the wrong oil in it and it's getting changed.
My goal was increased fuel economy and I didn't notice any. Now we will see what the 10w-40 does. The F-150 and 4Runner are smoother and quieter on the 10w-40. Nothing scientific, just observations.
I've used a lot of 10W40 myself, the ones we get here are a semi-synthetic that's Euro A3/B4 and API SN. It's always been a smooth and reliable oil for me, and often on sale at good prices. Various flavours available: DuraBlend, Magnatec, Penrite, Shell HX7, etc.