Yesterday I drained the atf from our race car transmission, it only has a few runs on it, so I wanted to save it. I drained it into a small gas can, then later transferred it to a gallon jug. The gas can has a screen on the pour spout and I was surprised how much gunk was on the screen. I had rinsed the gas can first so I'm unsure where the stuff came from. The bottom of the trans case maybe.
I'll know more when we drop the oil pan.