I'll pose this as a generic question:

From a fluid quality perspective, which is better and why (or does it not make a difference)?

Every 30k miles doing three drain and fills of ATF resulting in fluid that is 80% pure (i.e. new).

Every 10k miles doing one drain and fill of ATF resulting in fluid that is 42% pure.

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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife