I was getting my oil changed at a dealership & I kept smelling a burning oil smell after oil change, my engine was also idling quite roughly, they use Ac Delco 5w30 full synthetic, I decided to change my own oil to save money & I bought Castrol GTX 5w30 full synthetic & an stp oil filter the car has ran smoother ever since it doesn't idle rough anymore either, I'm not sure what the problem was, but in my effort to save some money I have a car that runs a lot better, I drive a Chevy HHR with around 255000 miles on it.