Are you certain that's oil? Here's why I ask: any oil introduced into the exhaust system will come either from the combustion chamber or from the turbo. In either case the exhaust gas temperature will be high enough to burn it, hence the cloud of blue/grey smoke that'll obscure the cars behind you. So it seems nigh impossible that it's oil - there's no other entry point into the exhaust system other than where it'll be burned. However a byproduct of combustion is water, which will condense in your exhaust system & pick up particles of carbon. This is particularly pronounced with short trips. If the drains in your muffler/resonator are plugged, or if there aren't any drains, those short trips will cause moisture to accumulate without generating enough heat to evaporate it. So enough water sits there, mixing with carbon in the exhaust, till one starts the car & the black, soot-filled water gets blown out with the exhaust before it gets hot enough to vaporize.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf