Long shot reviving this thread.
Did you eventually find out what was the cause of the leak?
I'm looking at buying the 535, but so far 3 out of 4 motors I've seen were soaked in oil. It looks like the leak originates from the inlet manifold.
However, since blanking the flaps helped you only for 6k km, it seems to me like the cause is somewhere else.
Maybe a daft question, but is there suppose to be any oil in the manifold? Because logically thinking, to me there shouldn't be any (unless I'm daft and uneducated in this area of the engine).
So any updates on what was the cause of the leak?