Just for clarity.....All MERCON today means MERCON V, yes?
There is an LV and a "special version" of the stuff too, no?
Originally Ford said no to backwards compatibility, but then stopped making/licensing old Mercon and said V was fine to use - it's a semi-synth fluid and holds up pretty well. Just make sure you do a FULL flush to get rid of all the old fluid if changing over from Merc/Dex3.
Be aware that most of what I have seen for Mercon V is a dedicated product - some stores still sell an old Dex/Merc formulation that would require additional friction modifier additive (if you can even still find it) in certain applications. Note that MercV is not suitable for transfer cases - Ford released a new fluid just for that purpose.
LV stands for Low Viscosity - it is a full synthetic fluid and incompatible with Mercon V/Mercon.
Walmart Mercon V is great fluid at a great price ($1.50 cheaper than Ford Dealership - they sell Mercon V and LV at same price) and is licensed by Ford, not just "suitable for use". I can't remember if they carry gallons or not though - only needed 2 quarts for my P/S fluid exchange, as the trans takes LV.
Side note, I'm not really sure what Ford did in 09 to the 4R75E transmission that magically made it incompatible with MV, but I sure don't mind the extended service intervals afforded by the full synthetic LV formulation.