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High oil pressure mean the oil is thick and possibly not flowing as much oil because the pressure relief is working and the pump is less efficient. Thick oil doesn't lubricate as well but has a stronger film strength which is what protects the big end bearings.
Its a balance: you need thick enough oil (and thus pressure at the pump outlet) to protect the big end shells and other places where the film is under heavy load, but you need to make sure the oil is thin enough to flow and lubricate moving parts.
Dont ask me what the right amount of thick and thin is but my thoughts were that if you dont have a cooler and oil gets very hot ~140C+ then you need a thicker oil than even a W30 to maintain viscosity but as I have an oil cooler that limits oil temp to 110C the film strength of 0W20 is still good.
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