My local Firestone runs a "sale" on oil, filter, & lube, and put a blend of synthetic oil in my 2000 Deville. Isn't the motor oil specified in tne manual better than a blend containing synthetics?

i dont think you are supposed to run synthetic in a northstar engine

Synthetic or dyno oil doesn't matter as long as the grade is right. The synthetic will burn faster than the dino oil though, so you'll have to check it a little more often.