I have about 600 miles on my 1st run of GC in my 2003 Silverado 5.3. So far in city I got 16.7mpg(about the same as M1 10w30) and I just did a 300 mile road trip and got 20.6 mpg doing 85mph!!!! This is pretty good for a V8 at any speed let alone 85mph and some hills! My all time best mpg with this truck was 21mpg with Mobil 1 10w30. So I think I still get the same MPG using GC as I did with M1. Oil pressure seems about the same at idle and speed. So only a UOA will tell which oil is the better in this engine. I will post it soon!