Is it safe to use Seafoam Transmission Cleaner on a 222,500 transmission with an unknown service history?


Background
My tranny fluid was pretty gross. Brown-black in color and giving me somewhat hard shifts on the interstate. I've only given it a couple top-offs since buying the car in January 2010 - probably less than a quart total.
I figured it was finally time to flush it.

I bought the IPD flush kit as well as a can of Seafoam transmission conditioner. ...and then I read the warning about flushing grungy fluid wrecking the gearbox. IPD has a warning on the kit and MVS has a lot of warnings about the entire enterprise - sometimes recommending that one leave the old fluid in...

I opted to drain the 3 1/2 to 4 quarts of fluid from the gearbox and refill. According to one set of instructions, doing this multiple times will eventually give me a full flush...

I prepped for the drain and fill by driving the car at operating temp, and driving for a bit in the three forward gears plus a few reverse. I let it cool off, and then removed the gearbox drain plug.

Almost a gallon of extremely nasty warm brown-black fluid poured from the transmission. This stuff had a tinge almost like old boiled brake fluid. I didn't see any floaters, but I did remove a fair amount of magnetic gunk from the transmission plug.

I was quite worried that my gearbox was going to die. But, after refilling it with Valvoline Dex/Merc, I took it for a drive and was amazed at the smooth shifts and generally improved feel. It's crazy how much better the car is moving.

So... given all this, should I use the Seafoam?