Welcome to the forum!
The oil/moisture is likely coming from the crankcase to the pipe and should go away when the engine is up to operating temperature. After sitting for so long, any oil that was in the crankcase and on the crank has settled to the bottom of the crankcase. When you started the sled, the air/fuel mix picks up that old oil as well as the new stuff. Oil doesn't burn very well, so it ends up going into the pipe. A cold pipe will cause the oil to condense on the walls, and then drip out. A hot pipe will keep the oil in the exhaust gasses and it exits with the exhaust.
I wouldn't worry too much unless it's doing it after you are up to operating temperature. Then there could be an oil injection problem with the pump, such as a mis-adjusted pump or one that is injecting too much oil. In that case, you would probably be fouling plugs, as well.