I'm writing this for a friend - I don't own a BMW and don't know too much about them.

Her 318ti was diagnosed as having a "bad rear crankcase seal." Is this similar to a bad rear main bearing seal? It has had a sudden rash of leaking oil. And the idle is now rough (is the crankcase somehow connected to the intake manifold? - maybe there is a pvc between the crankcase and intake manifold?).

If it is a rear main seal, how much to fix?

Thanks in advance.