When I rebuilt my 727, I had the pan back off to change converters after 30 or 40 passes down the track. It had a pretty liberal amount of "stuff" in the pan. I was told that was normal for a new rebuild. I put it back together and ran another 50 or so passes and took the pan back off. Still had a bit of metal filings in the "mud" in the bottom, but nothing like the first time. I ran another 200 passes and took the pan back off and there was a small amount of "stuff" in the bottom, but much less than before. It looked to me like the parts were just getting broke-in with 300 passes. This was in a very low HP, heavy bracket car, YMMV.

Danny