Pull the cover off of the engine. Just pull straight up at each corner. (Make sure all the rubber goodies are in place when you replace it.) You'll see the oil filler cap attaches to a plastic filler neck. Very common for the filler neck to get leaky at the bottom where it seals to the valve cover. It is easily removed by turning, just like the filler cap, after releasing the small lock tab on the driver's side. Replace the gaskets for the cap and the neck. I think you can buy them from Fastlane.
Good Luck, Dan

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2012 E350 Bluetech