There are corner cases in which the transmission clutches are so far worn that the uhh... crud suspended in the fluid keeps things moving. That can't possibly last for very long. If you're running the car into the ground then leave the transmission as is. If you're going to rely on this car for a while, service the transmission and bite the bullet in the unlikely chance it's so worn that it's hanging on by dirty fluid.

How does the fluid look now?

Sixto
87 300SDL