While driving at highway speeds for a while, my non-turbo 300TD wagon leaks enough oil to burn off a fair amount on the tailpipe, causing a plume of smoke behind the car.

Parked, doesn't leak a thing, other than the leftovers that drips off the crossmember and transmission from the drive.

Degreased the engine, and there's no obvious leaks, so it's hard to tell where the heck its coming from. Has anyone had this issue? I'm guessing it's a seal that gives to the higher oil pressure of a highway drive, but after that, dunno... Thanks for reading-