I'll preface this comment by stating I'm not a transmission rebuilder, converter salesman, or even familiar with HP Tuners. However, I have fried over a dozen transmissions as if it's some sort of natural ability- how nice I am to them doesn't seem to matter because even new ones have refused to function properly. Over the years I've gained a little insight from personal experience, witnessing issues in other garages, as well as many hours of research. I know absolutely nothing about your 6L90e, though I've never seen a factory GM O.D. trans last 200k miles. Hell I never see them clear 150k, and they seem to start going south rather quickly after about 110k. In years past 100k meant the car was trash so the Big 3 use that as a point of "good enough": Today's engines last longer, but once a transmission hits triple digits the safe play is to start saving for another.
In my experience putting fresh fluid in at this point could possibly usher that transmission right into an early grave. It's my belief that if the fluid isn't changed religiously every 25k miles (or sooner), and if it clears 100k without ever being changed then you better just keep using that fluid. Automakers know that most consumers don't keep up on transmission services yet still expect a car to last 100k miles, so the transmission will normally last about that long provided it's been kept full of fluid. I know it sounds crazy, but I've killed multiple transmissions by simply putting in fresh fluid and a filter, and I'm not the only one. The super fine sediment is actually helping the clutches grab right now, and once you replace it with slippery new fluid with friction modifiers you might end up with a "box of neutrals" due to their inability to grip anything. The gaps between clutches/steels, pressure plates, and pistons are bigger and the pressure is lower so adding a higher viscosity fluid such as Type F or Lucas may help, but it will affect transmission performance across the board and not always favorably. "Not favorably" is especially true in the newer electronic transmissions such as yours. Also, if memory serves correctly the black stuff is carbon clutch material from the forward gears and the glitter is brass torque converter material. Converters contain lots of brass (brazed fins and all), the only source I know of inside the trans is thrust washers and I'm pretty sure they're all plastic now.
It's worth noting that this exact issue has occurred to me mostly on GM RWD 4spd O.D. trannies which are notorious for having internal valve body leaks/issues, especially in the 3/4 clutch system. The (post '99?) 4l60/65/70e series with a continuous slip converter REALLY needs the fluid changed often. Even the 6l80es eat converters, especially when large wheels and tires are mounted to stock trucks. When the '65e converter in my '04 Sierra "locks up" or fully engages it's programmed from the factory to never do so 100% so its common to switch over to F Body converter tables and use aftermarket converter that's designed to be slammed On/Off. Hello "5th gear". Changing the converter duty cycle via computer and performing a throttle valve & boost valve swap helps remedy poor shifting and converter issues.
For a new converter I recommend Yank, Precision Industries, or Circle D- they get nothing but high praise from all that use them, and it's probably because they're very thorough and make high quality products that really perform. I spent $700 on a budget friendly one from Circle D last summer but haven't ran it yet. With 6 forward gears you can go stupid high on the STR, which means you can have a 4k stall converter that drives perfectly on the street both in town and on the freeway. A low STR would only benefit you at high speed upper RPMs, and would cost you sluggishness off the line. It works for drag cars that run out of steam at the big end by slipping less up top- again at the cost of low rpm responsiveness. A high STR will allow you to drive normally as if you had a stock converter but will still flash stall at the appropriate RPM when you mash it. The issue here has always been a shortage of gears to make the fun last- a TH350 with a high STR would be great stoplight to stoplight but would fall on its face at the track if built for torque. Three more gears means you can jump off the line and still cruise long distances without frying your converter.
For this transmission at this time perhaps the nicest thing you could do for it short of a complete overhaul is install a budget converter such as Allstar or Dacco, a Trans-Go shift kit (avoid B&M), and a new separator plate that doesn't have worn feed/exhaust holes. If that thing has 180k miles on it I bet the plate has been beaten to death and the BBs are trying to go through it. Doing these things will decrease leaks, restore line pressure, and even redirect fluid paths. The budget converter isn't necessary but your stocker is going out and wouldn't make good use of the shift kit, and spending $1k on a good one to put in a failing trans is a waste. Plan on spending AT LEAST $2k on a new trans, and another $1k on a good converter. If you can swing it invest at least 3 on the trans and buy the best converter you can afford. This isn't a place where being frugal has ever paid off for me. Ever. Even buying things that you'd think were all the same. The biggest lesson I've learned about transmissions is that good ones are expensive one way or another.
When it comes to this sort of thing you need only look to one member here: Hutch. The man is on the cutting edge of development in this arena and possesses knowledge that is only shared by about 3 others in the entire world. I'm definitely NOT one of them.
