Before you get too worried, remove your air cleaner box and the plastic plug wire shroud on the passenger side valve cover. Clean that thing up. Depending upon the amount of oil that exited the filler cap, the grooves alomng the top of your valve cover may be full of residual oil and that stuff will leak and drip for weeks.

I speak from experience... I had the same thing happen and it was pooled up oil sitting in the grooves/channels of the valve cover.

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Jeff
'87 300E
'92 400E
'06 ML350