A lot of W123 owners (me included) use Maxlife Dex/Merc ATF. If I recall it's on the thinner side of the spectrum of compatible fluids. Changing ATF's back and forth dependent on the temperature is senseless, but switching to Maxlife for good probably wouldn't hurt it any.

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".