I use Mobil 1 synthetic ATF in my 300E. No placebo effect here - it definitely shifts better. I agree that both the gasket and filter should be changed. You're in there already - why not?

As for the copper and aluminum washers, they are almost free from the dealer. I have a whole bunch of them strung on a wire and I think it cost me less than $3 for all of them.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280