got my first oil change and gas milage took a nose drive

[bighaas79]

For model year 11 and newer, I believe all gm vehicles use gm dexos oil for best mileage and emissions. If you look in your owners manual it will tell about it. Now it will also say that if dexos is not available that 5w30 can be used as a substitute. Does this affect milleage and what not? Couldn't say. I'm gonna say they switched to this oil just like ford came up with 5w20 a while back to be able to sneak their ratings for mileage and emissions up just high enuf to get past their required govt. ratings. I'm curious as to why you let the dealership change the oil if you turned around and put your own oil in it a day later? My guess is they would have used the dexos oil. stuff isn't cheap either. You probably just drained about $50 worth of oil out of your truck.