I can't locate my original thread for my Getrag transplant in my 280SL (not that I tried very hard, honestly). But I decided to get her out and drive her a bit, about two weeks ago. Interestingly, I could barely shift the transmission at all.
We had originally (and mistakenly) filled the Getrag box with transmission oil (like 85-90 I think) and it was unshiftable at all) We drained that out and put in ATF. That made it driveable, although a bit stiff. When I went to drive the car recently, it was miserable, but I got it out on the road, slowly, and warmed up the gearbox, and finally it loosened up. We decided to drain the box and refill it with synthetic ATF- I tried to get Redline, but evidently there's a shortage of Redline Dexron 6, so I got a different synthetic.
It's a significant improvement. The box still feels a little notchy, but overall it's MUCH better. I suspect that there was still some 85-90 left in the box from the first fill, even though we had warmed it up and drained it back then, and that it shifts better now because what's in there is only Dexron 6.