Ummm, what? Oil only flows in the rocker arm area, not through the head in any way. Coolant occupies that area.
Do you have a pic? Small block oil sending unit is in the back of the block by the distributor and a big block is just above the oil filter.
Could the valve cover be leaking? Oil flows down, not up so the oil from the valve cover could make the plug look like it's leaking oil. Just a thought.