I would add engine oil dye and drive the truck for a week. Then blacklight the engine looking for the tracer dye. I did find this bulletin pertaining to the oil sender leaking.
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
On rare occasions, an engine oil leak may be noted just below the PCV port between the number 3 and number 4 intake manifold runners. This may be due to oil that settles in the bottom of the intake manifold due to an internal porosity concern with the cylinder head that can cause oil to enter the PCV system.
If the SI diagnostics do not isolate a cause for the above leak, inspect the engine for excessive crankcase pressure by using a manometer and check for restrictions in the PCV system. If there is no problem found, replace the cylinder head for evaluation.
Please follow this diagnostic or repair 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.
Some customers may comment about oil spots on the driveway or an apparent oil leak. Upon investigation, the technician may find that the leak is coming from the area of the oil pressure switch.
The cause of this condition may be due to an incorrect type or application of sealant.
Remove and re-install sensor with new thread sealant. When re-installing an existing sensor or installing a new replacement sensor, the technician should be sure that the threads are clean and dry. Apply sealant to the threads in the following manner:
Notice: Failure to properly apply thread sealant may lead to repeat leaks or the failure of the oil pressure switch.
(Canadian P/N 10953489)
Thread Sealant - Loctite STS GM6124M
Parts are currently available from GMSPO.