What causes oily residue to form on the inside edge of the tailpipe?

One of the by products of combustion is water, when you start the car, the exhaust pipe will be cold, the water will therefore condense from gas on to the cold metal of the exhaust ( you can often see cars will water dripping from the exhaust on cold mornings!). Oil will also do the same thing, especially some of the thicker parts of the diesel.