I know it has probably been discussed a million times, but when I tried to search, I came up empty.
98 Deville, 60K miles. It was serviced regulary by a dealer, but I don't know what type of oil they used. I would be willing to bet it wasn't synthetic, but don't know if it was 5W30, 10W30, 15W40, etc.
I also need to know if these engines require just an "API" rating or the API with energy conserving values such as Penzoil. I have always used Penzoil in my cars, but never had a N*