I use Mercon V ATF fluid (what Ford recommends) and I substitute 1 quart of ATF fluid for a quart of Synchromesh. I've found that this is quite a popular thing to do in the TR-3650 and in many other vehicles.
You'll have to special order it as far as I can tell. I just have a couple quarts on hand that I use, but I have no idea where they came from.
Make sure you get the "friction modified" version.
A quick Google search turned this vendor up;