I would first remove the shield under the engine, takes about 30 minutes. Then you can asses the situation better. Wipe all the oil down good and clean, start the engine and see if any oil comes out. Then start the process of elimination...I agree that it may be the filler tube. But you never know. Be patient and check everything.
I went back to your other thread to see how many miles you have on your Boxster...74K. I would recommend the following maintenance's to ward off any costly fixes. I would do the TIP service, replace the fuel filter, replace the spark plugs and tube o'rings, and the AOS, plus it might be wise to do the water pump. I also have a 2000 and have done all this except for the water pump. Replace the serpentine belt if it is original for sure.