Put the top back in service mode and have a look where the oil filler tube comes through from the coolant tank area. You may find that the section of filler tube inside the engine compartment has detached from the coolant tank manifold and can be put back on with a new hose clamp, or it has actually split in which case you will need to replace it. Part # is 996.107.254.03. Pelican has 'em for $43.75
http://www.pelicanparts.com/cgi-bin/smart/more_info.cgi?pn=996-107-254-03-M100&catalog_description=&Oil%2520Filler%2520Hose%252C%2520Filler%2520Tube%2 520to%2520Lower%2520Oil%2520Filler%2520Tube%252C%2 520Boxster%252FBoxster%2520S%2520%2528%2531%2539%2 539%2537%252D%2530%2534%2529%2520%2520%2520
If you replace it, some will recommend that you replace the hose clamps with the screw kind. I bought the proper tools to work with the factory clamps instead. It's a personal preference thing.