If your bearing clearances and sizes support it, lighter weight oil is free horsepower.

I have run into issues on OEM stuff where if the journal area is marginal for the load on the bearings, that heavier viscosity oil when at temp is needed to keep the bottom end safe. That's one of the reasons why a bunch of new cars require oils with a wide spread, like 0w40 or 5w50.

I run 15w50 Mobil 1 in my motor because it's cheap at Wal Mart and has enough zinc for flat tappet cams. I'd prefer a 5 or 10w40 for cold starts since mine is mostly a street car, but my wallet prefers the 15w50. lol