I just found out that when my tranny was installed there was dextron III/mercron put in it, but it already had ATF+4 in the torque converter, and one of the fittings on the heat exchanger is slowly leaking so I have been adding Mopar brand ATF +4 to it.

Do I need to flush it immediately?

If so will it be alright to drain the pan, change the filter, fill back with ATF +4, disconnect the return line, and start and fill.

 

I wouldn't worry about it one bit but from now on use Dexron,
way cheaper and works just as well.

 

They are not compitable. They have different charactistics.