Oil in Lower Intake Manifold

PCV valve?

Just soak up the oil. What you have is not that abnormal. You probably make a lot of short trips which creates more moisture in the crankcase vent system. This moisture takes some oil mist with it through the PCV system and into the plenum. This is an area the customer isn't supposed to see. You might reduce the oil mist amount by switching to a 10W30-40 instead of a

If that bolt and the one diagonal from it don't have the threads sealed, the manifold vacuum will pull oil up past the bolt/threads and it will pool.

Worked on a 3800 a few years ago that had the lower intake gaskets replaced at a dealership, within a few months, the car ran like crap, in the process of diagnosing it, I removed the oil fill cap while the engine was running, huge suction (that shouldn't be). Pulled the upper intake, the bolt you highlight was just laying there across the low spot in the manifold. None of the lower manifold bolts had any sealer or Loctite on them. (draw your own conclusions)

I'd replace the lower intake gaskets. They fail just the same as all the other GM V engines except that the coolant passages are towards the center of the manifold, not on the ends so there is less chance of seeing traces of coolant leaking thus heading off a much larger repair.

On the upper plenum throttle body picture, see the three oblong holes adjacent to the EGR hole? Probe from the EGR hole towards the outside oblong holes with a pocket screwdriver, you're looking for any signs of the plastic softening or having broken away.

what kind of engine.

The oil or oily lookig stuff over there migh tpoint to leaky injectors. 4.3 vortec is famour for it.

It's a 3.8 L V6, the 3800 series II.

Like it wasn't obvious from his pictures...