Well, if all other possibilities have been eliminated, the obvious remaining answer must be true--the oil must be coming from the exhaust port area... somehow.

Perhaps there is an error in the casting, and somehow the holes for the studs penetrate an oil supply or return line in the head.

What happens if you temporarily remove one of the studs and operate the engine--does oil gush out? Do you have a good set of stud-extractors?

-Joe