What you are experiencing is a blown valve cover and gasket. I had the same issue. It is caused by a faulty Positive Crankcase Ventilation (PCV) non-return valve in the intake manifold. I had mine repaired back in February of this year to the tune of $1000.00. But, luckily for you, GM is fixing this issue at no charge to you. I just received a letter in the mail stating that GM is aware of the issue and is either offering the repair or reimbursement if you've had the car repaired. I would encourage you to investigate the issue before moving forward, but your symptoms are exactly what I was experiencing. If you feel you need to contact GM for further details, here is the number that was on the letter I received: 1.800.630.2438. You will need your cars VIN. Good luck and I hope everything works out for your vehicle.