Are you sure that it is oil, Sometimes water or condensation that builds up in the exhaust will color black and leave spots around, on top, or leak down exhaust pipes from the exhaust pressure blowing it out. I would make sure that there is a small hole at the lowest part of you exhaust system to drain excess fluid.
Otherwise, The only thing that would push oil out the exhaust is:
1. Crank case breather or hose plugged, Happened to me which forces oil past the turbo seal and out the exhaust pipe which causes smoke at idle.
2. Turbo going By By.
Just my $0.02.