Ok, What have I done now... (904 problem?)

Mopar to ya said
No, it won't cause it to fail, but the friction modifiers are much different and cause firmness of shifting as you say, and also slippage. This is an old tranny made to run on the thickest Dex 1 ever made. Now you have a fluid designed to work less with bands than with valving and is a much slicker fluid. There was no problem before, now there is. It is a simple thing to change the fluid and put the proper fluid in. If I am the owner of this car, I drain all the fluid, put the gasket in the right spot, drop in five quarts of Dex II, run it hot and check and add until full.

The difference is minimal. The viscosity of the fluid is even less of a difference. Auto transmissions operate primarily on hydraulic principles. Once the clutches are applied the fluid is squeezed out from between the steels & plates anyway as they grab. A thinner viscosity fluid would only enhance the precess. Viscosity has not that much to do with friction & protection. The clutch materials in the 904 are probably the same materials used in type 4 fluid transmission. Espescially if it was ever rebuilt. There are several guys on this site & this post alone who testify they have used the type 4 fluid w/o problems. There is most likely a mechanical problem that occurred during or coincidently with the filter & fluid change.