The AFM (automated fuel management) setup on many GM engines that causes the engine to go to 4 cylinder mode when less power is needed has caused a plethora of oil consumption problems. Supposedly, this problem got resolved with the late 2011 engines on, but time will tell. Many, like me, have effectively disabled the AFM from ever going to 4 cylinders. The engine remains in 8 cylinder mode all the time.
Now, to the oil problem getting into the intake from the PCV system. Simple. Most, if not all, folks should consider installing an oil catch can in the PCV line from the PCV valve/fitting and the intake manifold. This will filter out any oil that normally is drawn up with the crankcase gasses. This oil can reduce power, in that the ECM has to compensate and sometimes resort to low octane settings in the ECM. Also, this oil will coat the inside of intake manifold, add the EGR exhaust gasses coming in, and you have a recipe for serious crud. And some of this will coat throttle bodies and valves.
All in all, just a bad situation just waiting to happen. These oil separators are relatively inexpensive and not difficult to plumb into the PCV line. I just purchased a 2013 Silverado with 5.3L and an oil separator will be going on within the next few weeks.