I'm coming up on 350k miles on the original M46. I've had the overdrive replaced (used unit), but the tranny has never been opened up. How many more miles can I expect before I will have to tear it down? It is difficult shifting into first (reverse detent needs adjustment?) and sometimes "grinds" going into 2nd, but otherwise works perfectly.

I suspect part of the reason that it has lasted this long is that 90% of the car's mileage was done on the highway (not much opportunity for shifting). I also change the fluid every 25k.

Also, what usually wears out first in these boxes? Synchros, bearings, etc.?

TIA.