I have an '87 300E with 175K. Shifts like a dream. Had to reseal twice but otherwise I change fluid/filter every fall at the end of the hot summer. I believe that heat will also shorten the life. We live in the California low desert. 115F is very common during July-August.
Anziani

'87 300E 175K
'91 190E 147K
'95 E420 90K