Oil in the airbox on a N/A engine will be caused by one of two things.
Since you have the valve cover off is there sludge buildup?
If it is in the valve cover it is in the entire engine.
Reinstall your valve cover and start the car.
Remove the PCV valve from its grommet and place your finger over the valve, do you feel suction?
(You need to verify engine vacuum is correct at a manifold source and at the PCV valve)
Be sure none of the PCV/breather hoses are split/collapsed/loose fitting and the breather filter in your air cleaner is clean and not restricted.
If all of the above are ok....
As stated above by midnighttoker remove the oil cap and breather tube from the air cleaner with the engine running.
If there are signs of smoke the problem is internal engine, usually rings.
A compression and cylinder leakdown test will be needed to verify.
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