Got pulled to this from another oil comment I made earlier today.
I worked at an automatic-transmission test facility a number of years ago, and we learned something funny about Dex 6.
We typically ran tests in 1000-hour increments, with an inspection teardown at each interval, and then re-test on fresh fluid.
When GM introduced Dex6, we used that when called for. Sometime into test segment #2, we started having leaks at the input and output seals, which had never been an issue before.
After lots of evaluation by lots of folks, the chemistry guys determined that the Dex6 had a higher level of X than previous. X is used to keep the seals soft and sealing, and it deteriorates somewhat over time. So when we did the fluid change, the trans (and existing seals) got a fresh slug of higher X, and it went to work softening the seals even further, causing leaks. IIRC Dex6 got revised to correct that.
And from my discussions with teh chemistry guys over the years - don't mix fluids. It might be OK, or it might not, but if not, you won't have the resources to diagnose it back to the fluid soup you used, so you'll likely make the same mistake again.