transmission oil change

[yarikoptic]

Dear Subaruers.

I had a differential binding problem, so I've changed all tires so to make sure that all of them are equal, did a bit of rear-8-figure-driving and changed AT oil. I thought to change AT filter but I couldn't take the pan off because of the excessive rust -- didn't want to bother this time.

The problem/questions are:

what is the amount of AT oil usually gets dumped when no filter gets changed (damn me - I couldn't measure -- trusted too much to the manual). Manual says 4 Qt, Friends says 6 Qt. I added first 3 Qt, started - couldn't get stable level, added another 1 Qt, oil seems to appear but too many bubbles, added another 1 Qt, seems to be much better but few bubbles
Rear -> Drive -> Neutrual -> 2 minutes, it is hard to get stable level but it seems to be on Low Hot when engine is hot

how dangerous to overfill? all official sources say that overfilling is very bad, friend says that not really

appearance of bubbles mostly due to over or under fill??

how bad to mix different brends of AT fluid?

Thank you in advance for clarifying the questions for me.