[ppbon]

... inspect carefully.
First check around the oil cooler to make sure that that's not where the oil is coming from. Inspect carefully with a flashlight and a small mirror, so you can see behind and under the cooler.
Then inspect the oil filler tube itself.
They tend to crack and will leak oil on top of the engine when:
a) you put oil into the engine through the rear trunk's oil fill
b) you generate enough revs and motion in the car so that oil creeps up the tube
Happy Boxstering,
Pedro