So I know everyone had their own opinions on oil, but I have a question. We purchased a vehicle care plan with my 2010 Silverado which covers oil changes and tire rotations. I was told the dealership I go to used Castrol GTX but found out they use Valvaline. I've read that Valvoline oil leaves a waxy residue in engines and that it's not a very good oil. I'd rather use Mobil 1 synthetic like I have been for that past 8 years, so if this is true about Valvoline, Ill just go the oil changes myself.