what type trans fluid is everyone using

My 65 Dart 904 slips a little from 2nd to 3rd, not to bad. My question is could I drain pan and replace fluid with Dexron would that add a little friction and help? Or would that just muck it up and be worse? I've had 904's rebuilt and they also had me use Dexron over ATF fluid saying it was to slippery. This is older and had sat for a few years, should I do it?