I did have an engine that was pushing oil temps in the pan to 280 - 290 F on long interstate runs. Oil breakdown was a concern - I was using racing oil - should have been using synthetic racing oil but thats another story...
My understanding, mostly from Widman's paper, is that high temperture high shear conditions break down oils. High temperature alone isn't a huge problem until the oil is pushing something like 300 degree. Generally oil temperatures are close to coolant temperatures, but on my Barracuda I installed an oil temperature gage because it was a specific concern with that engine setup.
Viscosity at 160 to 170 can be significantly higher than at 200 F. If you want to run colder engine temps, the target viscosity can be obtained by using lower viscosity oils. But the lower viscosity oils tend to have poorer High Temp High Shear which can be imprtant with high valve spring pressures. As usual trade offs need to be balanced.
The other reason to get the oil up close to 212*F is to get rid of the condensation that formed inside the engine.
Here's viscosity vs temperature for four grades of oil listed in the '68 FSM.
View attachment 1715600978
This illustrate the difference on viscosity between an oil at 160 F and 200 F.
The 5W-20 was only for use in cold weather - consistantly under 10* F. Its probably safe to say that the minimum acceptable viscosity was around 8 centiStoke.
10 to 20 cSt was probably the target operating range.
