Purchased a 2014 chevy 2500hd with a 6.0 engine with 90,000 miles on it. I knew the truck and all are hwy miles the old owner had always changed the oil with mobil 1 5w-30 full synthetic every 5000 miles. Iv done two oil changes with the same oil in it and before i get 3000 miles on it the oil gets really really really black. Has anyone ever had this issue before? Engine runs great!