you wont lose more than half a quart or upto a quart when you undo that, the only fluid that will drain from there is whatever is in the ATF cooler and line.

the bulk of the fluid is in the pan and torque converter of the transmission which you are not disassembling.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)