Anytime you have forced induction on a car that was not originally designed for it, you increase your chances of random oil leaks.
I would definitely check the PCV system, they are known to have issues, I just changed my wifes cars whole system about a month ago.
Also, I had a crack in my valve cover on an old 323ci that I had. I don't know how the crack developed, but its possible. The joys of plastic valve covers.