Tim, let me throw some ideas out. Im sure you or your mechanic checked, but ill still say what came to my mind.
1. Is the crank scored where he seal rides on it? 2. Are you sure it is not a oil leak from the turbo base?
3. Are you sure the leak is not diesel fuel from the lift pump?
A scored crank just won't hardly seal. There are two o rings between the turbo and the intake. Those o rings can dry out and crack causing a major oil leak. I've also had leaks I swore were rear main until a closer look (smell) revealed a leaking lift pump.