High mileage motor oil

Thinner oil runs out of the bearings faster which results in lower pressure. With high mileage you get increased bearing wear and therefore greater clearance and it's even easier for the oil to run out. You want thicker oil, meaning a higher viscosity. 10W-40 is likely already thicker than what was originally called for so I'd stick with that. That will find it harder to ooze out of the bearings so oil pressure will tend to remain higher, which is desirable.