Assuming it has the Mitsubishi FM146 trans (which is most likely), it's specs call for 80W gear oil (GL-4).
You are correct, Mercon V didn't come about until around '95 or so if I recall right. My experience has been not to use it in anything that doesn't explicitly call for it even though Ford now wants everyone to believe it's fully backward-compatible with Mercon... Seems it's too slippery for the synchros in the M5OD manual, and causes hard shifting.
Mercon fluid is now called "Dex/Merc" or "Multi-Vehicle ATF" because of this. If you do happen to have the M5OD, this is what to look for (applies for '07 & older models w/M5OD).
