When the transmission shifts gears the clutches slip slightly. Otherwise where would be a harsh slamming shift. Like a transmission with a racing shift kit. When this happens some material from the clutches will wear and end up in the bottom of the pan. This is normal. When mine was my daily driver I changed the filter and cleaned the pan every year. I found the grit/ sludge every time. After 19 years it's still working fine. Unless you had a huge amount I wouldn't worry much.
I had the fill tube grommet replaced this week while I also had the trans fluid and filter changed......also had any other exterior gaskets/o-rings replaced like for modulator, etc. while it was in transmission shop.
Tech showed me I had some grit in the bottom of the pan when he had it down.....my question is, after 2,000 miles of dusty off-road use in south Texas over the past 2 years, shouldn't I expect some grit in the pan from all the dust and sand I've been driving through? Or is something more serious afoot?