There is no tranny pan to speak of. The oil pan is cast aluminum, so maybe that is what you are referring to. In any event, the grime accumulates at the seam between the engine and the transmission. There is a small black rectangular cover at the seam, facing toward the engine. This is the area where the grime shows up. My van is in the shop right now having a number of problems addressed, including the oil seeping out in this area. I don't know if it's motor oil or trans fluid, and right now they don't either. They have a number of theories, but none of them sounded good to me. When they find the source definitively, I'll post it. It may take weeks, though, because the leak is very slow. They washed off the grime and want to wait to see where it comes from. I have a feeling they are going to end up pulling the transmission in order so see. There has been an issue of engine block porosity and cracking on the torque converter side of the engine block, at least that is what they told me. They want to see if that's it. If it is, I'm gonna raise a bit of hell if they want to swap the short block on a new (2600 miles) 25K vehicle.