SOMEBODY put the WRONG trans fluid in the trans

1988 Ranger Custom inherited from father-in-law.
The Haynes repair book says there were 2 transmissions made for this truck.
Mazda M50D which uses MERCON Auto Trans fluid.
Mitsubishi M50D which uses SAE 80W standard trans lubricant.
The Haynes book says I have the Mitsubishi Trans BUT THERES MERCON IN THERE.
A couple of years ago I went to a Nationally known 'Quick Lube' place and of course they said it needed to be changed, which I said ok to.
I've also taken the truck to the local Ford dealer and had the oil changed, and some repairs done.
So what do I do - is the wrong stuff in there? - There is NO drivability issues, it shifts smoothly, no noise, nothing that I can think of that's wrong. I'd just like to use the CORRECT FLUID and if possible that it is the gear lube that's the right one I would like to use a synthetic lube as I don't drive that many miles per year (I've also been using Mobile 1 in the engine) The photos enclosed are of the label on the drivers side of the trans.
and of the decal on the drivers door.
Thanks for your help, Patrick-OrOldMan