When I bought my Tahoe, I drained everything. I always assume that the previous owner did not do anything about it. And I did well because I realized that the transfer oil was not the right one.
For the automatic gearbox, I make three partial emptyings with 600 miles of interval, like that, I remove as much as possible. At the last drain, I change the filter and I go on something clean.
Like many American owners, I do my maintenance alone, especially on the automatic gearbox, because in France, the mechanic is not trained on it.
The French or German manufacturers had released the concept of "gearbox without maintenance" ... result: the gearboxes automatic were good to change after 60,000 miles. Few companies can redo an American box and to redo a simple TH700R4, which I had on my Firebird 85, I had taken $ 1700