Paul,
I tried Type A ATF, Dexron III, and Redline D4 Synthetic ATF. Didn't notice any difference between Type A and Dex III, but the box shifted slightly better with the Redline D4. After rebuilding the 'box I put in Dex III, it works fine. I'll still switch to D4 before long, but overall it doesn't seem to make much difference which type ATF you use. I tried gear oil, too, and that was definitely not the way to go, so hopefully switching to ATF will improve shifting for you.
George Davis
'69 280 SL Euro manual