When my dad bought his 2500HD Silverado with the 8.1L gas V8, he was using about 1 qt. every 1k miles. Chevy said this was normal, and for a large displacement V8 like that, it can be. He was running Kendall GT-1. Back then, I was an Amsoil fan, and recommended he switch to Amsoil. He did, and his consumption stopped. But today, I would not recommend Amsoil. I would recommend something like Rotella T6 or Delo LE instead. They are found at many more locations, cheaper, and the performance gap between Amsoil and the competition has shrunk considerably to the point where the $$ and the hunt to find it aren't worth it.
YMMV