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.