Are you running a PCV valve? If not, try it out. If you are, change the valve. I'm thinking that you're having pressure building up in the crankcase, pushing oil out of the crank and pan seals.
Great point, this is often overlooked. I just had the same issue on my new little roller 318, changed the PCV valve, leak stopped.
If you run a HV oil pump, it even makes it more difficult on some of the sealing.