why do i have oil under the plenum

What is your question?

i have oil laying in the valley between heads under the intake. i found what looked to be a 1/4 inch rubber plug laying in the oil. also a walnut yes a walnut. i dont know it got there. truck has been garage kept from new. also when engine is under a load such as pulling a hill or under full throttle. also check engine light is on.Had that scanned i came back as small evap leak This a started about the same time

How long has your 2010 Ford Explorer had this problem?

about 2 weeks