I've gotten into the habit of checking the flatness of the pan mating surface with a straight edge every time I drop the pan. It shouldn't be difficult to straighten out a pan on the bench with a hammer backed by wood or metal.

I too use the cooler line method of changing transmission fluid on my cars... but not on my MB, because it has a drain on the torque converter, which allows me to drain >80% of the fluid in the transmission unit.

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95 E320 Cabriolet, 169K