My 2009 E350 4matic wagon manual says to change the transfer case oil and transmission fluid at 40,000 miles...BUT...when I took it to a local certified indie mechanic (good reputation) he told me I didn't have to do the tranny fluid until 100K miles and don't do the transfer case oil unless there's a problem because "it's a sealed system and there's no need to go opening it up unless there's a problem". Is this correct / good advice? I don't want to spend $ unnecessarily, but if it should be done I'd rather pay now than later. Any help is appreciated.

My indie, who is a great guy, told me the same thing, saying that the synthetic fluid will last the life of the transmission. So it looks like I will be off to the MB dealer for the transmission service. I am also one to believe that changing the power steering fluid is a good idea, especially with the rack and pinion system giving the fluid a workout.
Mike T.

i'd change the tranny fluid at 40,000 miles and the transfer case at 40-50,000 miles.

Thanks guys. I will get both jobs done. The mechanic may be right, but it certainly can't hurt to have it done. Power steering fluid sounds like a good idea too and I'll explore having that done as well.

Any idea how much each of these jobs should cost?

ATF does last a life time. When the tranny dies, the life time is up.
Check the TSB. I would change the fluid at 40,000 miles.