I am having the same issue, oil in the coolant on a 2016 5.3 Silverado LTZ 1500 4x4. The truck has 53,500 miles on it. Dealership can't find the problem. They flush the system, replace the antifreeze, add oil to the crankcase and tell me to drive it for a 1000 miles. I drive it about 10 miles and the coolant reservoir is full of what looks like coffee with a lot of cream in it. It is back to the dealership for the third time. The first two times, they couldn't find the problem. The third time the service manager said they would have to "dig a little deeper". I am not very happy right now. I love the truck but I'm not sure if I want to keep it.