Date registered: Feb 2008
Vehicle: 1987 420SEL
Location: Ontario, Canada
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Change the fluid and filter every ~25,000, (or whatever it states in the owners manual, I forget what it said) and you should be trouble-free for at least 250,000mi.
The key is to use good quality ATF, like Mobil-1 synthetic or AMSoil, and get that filter changed with every fluid renewal.
A rebuild for one of these runs ~$1200-$2500. There are a few replacements you can do yourself, like the B2 piston. Do a search in the W126 forum for more information.