Thinnest oil that gives acceptable hot idle oil pressure, within reason.
GM went on a big crusade decades ago to get people away from 10W-40. This was beaten into our heads at the GM Dealership back in '84, and it wasn't "new" then, although I think it was new-ish.
The bearing has better load-carrying capacity when the oil clearance is less; and tighter clearance requires thinner oil. Thinner oil flows better when the engine is first started. It's like a three-way win. Building engines with oil clearances you could push a kitten through is obsolete, if it ever was a recommended procedure.
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