The only reason to not change a lubricant/working fluid in any hydraulic system is if you have some type of renewable filtration system (a pan filter definitely is not a decent filter system). I had the fluid changed last year at 110K on our 98 ML320, the old fluid was black and no doubt in need of changing. Transmission smoothness no doubtedly improved afterwards. We are up to 125K now, hoping to get another 12 months of use out of the ML before we sell it and get a 2003 or 2004 ML.

Had my MD dealer do a "Transmision Service" at 150,000KM which included fluid and filter. Will do again soon at 250,000KM.... and hope to do it again at 350,000KM original transmision cross my fingers . Drives smoother after the service no question.

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1998 ML 320
BIG Sound