Ok, it's a dog-pile so I'll jump in. I've always believed you shouldn't mix transmission fluids. I believe that a blend of type F + at least the older Dextron would cause foaming.

Side note - Someone could run [u][/u]completely type F if you drained the converter and everything + type F has a little higher friction to help older transmission shift better & drag cars to shift harder + I think B&M's "Quick Shift" stuff is type F with blue dye.

To the original poster, a very reputable transmission shop owners showed me a technical bulletin from Shell Oil. It said that if you hadn't changed your transmission fluid in a long, long time, you should only replace the pan's fluid (maybe half the fluid?) + filter and don't flush the system or do a complete fluid change.

Fluid changes over time and it's solubility changes. Waxes come out of the fluid and start to collect in the transmission which isn't much of a problem with the old fluid in there, but if you put 100% new fluid in the transmission, all these waxes will really come out of solution and start to gum up the small passages inside the transmission. Keeping a good portion of the old fluid in the transmission helps keep these waxes dissolved in the transmission fluid where they do no harm.