ATF, like manual says. But I'm thinking of using HD silicone spray in addition to ATF.

I've read a bunch of threads as to what different people use. Sounds like general attentiveness and thoroughness and frequency may be more a factor than what's actually used.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

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