Here's a thought on oil viscosity

There is more to it than that. It's a different story if you measure the oil pressure at the last place to get oil. That would be the end of the rocker shaft on the number 7 cylinder.

I've seen virtually zero oil pressure there with 40 pounds on the gauge.

Where you measure pressure is more important than oil pressure. I won't run anything less than 50 at a hot idle if I can. That's about where I'm at now and that's with a 5W30 synthetic. I bought some rockers from a guy who said he was having issues with the rockers galling the shafts. As soon as I looked at them, I knew the issue and bought them for 25% of new. Polished the shafts and lightly honed the rockers. Looked at the shafts not too long ago. They looked perfect. The other guy had less than 100 miles on them.