We keep our 300SD parts car covered and in running shape (battery tender) and run it about 25ish mins once a month or so, shift it into drive/reverse a few times....rev the engine to 2300rpm for 2-3 mins with a full electrical load (all accessories on).......seems to keep it in good shape.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)