oil on spark plug coils

As a new member, I ask for the wisdom of the more learned members. In the process of changing my spark plugs of my 4 cylinder 06 Tacoma for a government emission test, there was oil on the coils. All spark plugs were free of oil. While the front coil was clean, they became progressively oilier as you went back to the windshield. What is the cause? Is there anything that can be done at home? Despite this, my truck past the test!

Thanks, misteryabut

 

Oil on the coils? Like on the boot where they sit on the plugs? Initially it makes me think valve cover but you'd probably see it pooling around the spark plugs unless it just started and it's slowly leaking. As for it getting progressively worse as they go back, bolts loose/someone did a valve cover and forgot to properly torque them.