It could just be oil in the plug well from a bad o ring seal on the valve cover. In that event the VC needs to be pulled and new gaskets installed. I'd do both if this is the case.
If that's not it then you've got an internal problem but I'd start with the valve cover.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.