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A lot of what is "best" depends on the bearings and how the engine was designed. Newer engines have different tolerances than our old 80's design.
When a bearing gets too hot it fails, oil flow does the cooling.
Too hot is the temperature where the oil actually starts to fail, not the bearing itself.
Thin oil may flow through the motor too well, so some parts of the motor lack the pressure to overcome the forces created by the movement of the parts like the crank, and starve some bearings.
Thick oil may not flow through the motor well enough, and starve some bearings.
Oil needs to match the motor design.
I don't know how much viscosity and film strength are related.
I don't think pressure at a bearing matters much if at all, just flow. I'm pretty sure a zero pressure oil source with a vacuum on the crankcase to pull the oil through the bearing would work fine as long as the bearing tolerance was sufficient for the flow and the rotational forces didn't effect it (no tying the feed to a rod bearing).
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