On our company van, a 1997 Savana diesel with a 4L80E tranny, I get 3 1/4 litres (about 3 1/2 qts) when I undo the drain plug. I do this every oil change at 5K mi intervals and only drop the pan at 50K mi intervals. I do not even use synthetic ATF. The van now has 390K kms(about 250K mi), about 95% of it city stop n go driving and the tranny still works perfectly. It has never had a flush. I have considered disconnecting the tranny cooler lines to do a flush and also switch to synthetic ATF but why mess with success, I am going to keep doing what I am doing now.