There is no oil going through the lower intake manifold, but when the gaskets go, the oil or coolant can leak across the edges. Usually, since the coolant is at higher pressure than the oil, the coolant gets into the engine.
But check this out, which I found while researching this. I found this in the "Cooling System Description" section for the engine.
Engine Oil Cooler
The engine oil cooler is a heat exchanger. It is located inside the left side end tank of the radiator. The engine oil temperature is controlled by the temperature of the engine coolant that surrounds the oil cooler in the radiator.
The engine oil pump, pumps the oil through the engine oil cooler line to the oil cooler. The oil then flows through the cooler where the engine coolant absorbs heat from the oil. The oil is then pumped through the oil cooler return line, to the oil filter, to the engine block oil system.
So that is another suspect.
Please let me know what you find.
Take Care, Greg A.