Cadillac does
not specifically recommend 10W-30 (the heavier oil) in all Northstars.
Your engine might be spec'd for 10W-30 above 50 degrees, but
2000 and later are spec'd for 5W-30, same temperature.
Like ILSAC specs, GM specs vary with the seasons or barometric pressure
.
........And, in 2000, one running change was from "flat tappet" style cam followers to rollers, allowing lighter cold oil viscosities as well as accomodating the slightly reduced zinc/phosphate load in newer oils.