I believe galleries is the correct term here.
So you meant the oil galleries that deliver oil to the journal bearings and other parts? Did they give a specific reason why the oil gallery passages were reduce in size, and how that correlates to needing to use a thinner oil? I don't see how reducing the size of an oil feed gallery passage is going to dictate what oil is used. It might increase the oil pump's output pressure slightly if the passages are slightly more restrictive due to the oil pump still trying to deliver the same oil volume per revolution of the pump throughout the oiling system. If they designed the oiling system correctly the oil volume supplied to all pressure fed components in the system should still be well above adequate, even if the galleries are a little choked down.Thank you! That's what I was looking for! (Getting old is for the birds!)
The actual physical design of the journal bearings would be the main component in an engine design that may dictate what oil viscosity to use ... as when engines are specified to use 0W-16, the journal bearings may be physically wider to help maintain an adequate MOFT when under high loads. There is a reason that 0W-16 has it's own API designation, because it should only be used in engines specifying 0W-16 (ie, designed for 0W-16). You would not really want to use 0W-16 in any engine specifying xW-20 through xW-50.