I would try to hook up a manometer to the engine (off the dip stick tube might work) and see what you have for pressure in the crankcase. As mentioned, if you're only leaking from the rear main, I would suspect something wrong back there. Crankshaft seal surface good? If the engine has high pressure, like mentioned, might not be broken in yet. Perhaps broken ring, etc. I'd check pressure first so you know what to look for.