I don't think Mercon is a synthetic fluid. The V part is just the latest update or spec for it.
All you need to do is use a Mercon V spec fluid, it may or may not be synthetic.
Synthetic Mercon V from Ford last time I got some was like $9 a quart. In a black bottle.
90 LX, Previous 4 cyl.
408W In the Works
TW FAC's, Jay Allen cam, Super Victor
2001 V6 5 Speed Daily Driver
Windstar Intake, Dual Exhuast, CAI, Headers
Best ET: 15.44@91 mph (on 18's)