I noticed yesterday evening that when I took off the pickup smoked white smoke after it was warm but then cleared up. This morning I started it up and it ran fine, I drove the 8 blocks to work and parked. An hour later I went out and there was oil leaking out of the muffler. It is black and oily and has made a large wet place under the muffler. It smoked again this morning and it hasn't been doing that. What could this mean?