here is a tsb that sounds like your problem to me.
#PIP3380: 6.6L LB7 Duramax Diesel Oil Leak from PCV Vent Tube - keywords contamination crankcase fuel - (Mar 31, 2005)
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in the PI.
Oil leak from the PCV vent tube.
If an excessive oil leak from the PCV vent tube has been verified, check for diesel fuel in the oil.
If diesel fuel is found in the crankcase, complete the current SI diagnostics for Fuel Leaks Inside of Engine
If there is no fuel in the engine oil and the crankcase is not overfilled with oil, check for a sticking or inoperative PCV check valve. The procedure for checking the PCV check valve is found below.
Shut off the engine and wait five minutes to allow the decompression of the crank case. Remove the brass colored check valve at the base of PCV box which is a part of the alternator bracket. If a measurable amount of oil (more than a several drops) flows out of the PCV box replace the PCV check valve P/N 97302067. This is a one way check valve which should have a free flow toward the crank case when the engine is not running. Oil accumulated in the PCV box should flow freely into the crank case past this valve with the engine not running. When the engine is running crank case pressure may prevent oil in the PCV box from flowing into the crank case.
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.