Anyone know what this white thing is?
Feels like hard plastic, looks like some kind of internal seal but I have no idea.
The only recent change in performance (before oil change) was a couple of recent hard downshifts into second upon heavy acceleration that caused bucking - was exactly like if you drop the clutch to fast from a dead stop shifting into first in a manual trans but I topped up the fluid and then that issue seemed to resolve -- though I've not really tested it too hard.
Just finished the transmission oil change.
Drained 7 qts, dropped the pan, new filter & filter ring.
120,000 miles. I have 3.5 qts of fresh fluid remaining which i'll probably put in there in a few weeks by draining from cooler line.
See pictures attached.
Thank you!

That is the thrust washer is the between the second clutch housing and the drive sprocket support. These can often break and have no immediate symptoms. A piece of it can stay in place, or the drum can be riding metal-on-metal.
GM went back to a steel ones in the mid 90s because of this issue. There is no mechanical thrust loading on that washer, but it seems the mass of the clutch drum, in combination with normal bumping and jouncing and vibration is enough to break the plastic ones.
I learned this from an expert. I found mine broken 21K miles ago, but my pan was clean.One piece in the pan and one hanging in the filter.

Thank you! That pan IS clean! Based on your history of high mileage cars, I'm guessing your vehicles enjoy plenty of TLC.

I was hoping that that thing was part of an accumulator assembly - behind/above the pan and easy to access.

It has been approximately six years and 33,000 city-miles since I last changed my fluid. At that time, those donuts and the pan were about the same as they are now. That is not so good because I believe it was the first time the oil was changed (at 89K).

The drivetrain could probably use an overhaul but at the moment, that is not a part of the plan.

The previous owner of this car (an elderly friend) had told me, the shop that had serviced this car, had commented how little clutch material this transmission sheds. He had taken it to them a few times because of an occasional 'no drive' condition cold. The shop never wanted to tear it down for such an occasional issue (especially since it worked so well otherwise). It always resolved itself as the car warmed up. I have not been able to duplicate this again since changing it over (most of it, by now) to Dexron VI. We've put 73K miles on it since we've owned it (now at 259K).
Our '96 (4T60E vs the non-electronic version in the '92) has always shed so much clutch material every time I have serviced it. It concerned me the first time (we bought it new) I changed the fluid, but it is still working at 209K miles.

My new fluid is Dexron III (H) but i'll probably add a gallon of Dexron VI when add the remaining three quarts.

I'm a bit concerned about that second gear. I stepped on it this morning on my way to work and had a few more hiccups upon downshift into second - until I let off the gas. Would you happen to know if that issue might indicate a problem with the clutch pack or is more likely due to the thrust washer issue?

What were the hiccups?

Engine rev up then clutches engage?

Were you cornering at the time?

Any driveabilty problems or broken motor/transmission mounts seem to be badly amplified on a forced downshift. I don't really know if this could be part of the failed thrust washer.