The only thing "special" about the E300 battery is that it sits under the rear seat. So Mercedes specifies that it must have certain characteristics not to leak at all. Not all FLA batteries will meet that spec. If money weren't a consideration I would throw an AGM in there and forget about it.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife