Stock 390 engine in my '64 TBird with some minor build-up, probably turning out 350 HP.
I rebuilt the transmission two years ago and it runs perfectly after warm-up. Shifts are firm and predictable, much, much better than before. The rebuild kit I used had some sort of fibrous clutch plates- very different from the factory plates that I took out.
After not many miles the transmission started taking a long time to warm it up. Until warm it fails to shift into third unless I run it about 35 mph and there is no load on it. This issue issue got progressively worse, the warm-ups took 2 miles, then 4, then 6, then 8. I suspected that a cheap grade of Type F fluid was the issue, so I drained the torque converter and dropped the pan. I then found that the filter screen was clogged with clutch plate material. No metal. I installed a new filter, 11 quarts of Valvoline Type F, and that fixed the issue completely.
Apparently the clutch plates are still breaking in and shed material that clogs the filter, reducing pump pressure when the fluid is cold and viscous.
The warm-up issue came back. It happened more slowly than before. I planned, this time, to convert the filter screen to a pick-up and install a remote oil filter on the supply side of the cooling circuit.
So I dropped the pan and guess what? The filter screen was clean. But, a thin layer of black sludge on the bottom of the pan and filter. The fluid is more brown than red. I drained the torque converter as well to get rid of all of it.