Is it dangerous to pour a 'non original' transmission fluid on a Ford Focus 2 facelift (MK2.5) with a MTX75 manual 5/1 transmission?

By non original, I refer to a third party brand like Motul and Valvoline transmission fluids that meet the specification.

I researched quite a lot on the internet and an MTX75 5/1 takes 1.9 Liters of transmission fluid with a viscosity of 75W90 with the specification WSD-M2C200-C.

The lowest prices in my area are the following (shipping cost excluded):

Ford: 26€/L (22,26pounds)

Motul: 16,13€/L (13,81p)

Valvoline:15,32€/L (13,12p)

The reasons of my doubts:

1) Read/Heard on the internet that it's best to stick with the official fluid because that's the one the car has been designed to run with. 

2) The car is second hand and i don't know if the transmission fluid has ever been changed nor I have a way to find out.

3) Maybe more modern oil is better?

4) Huge price differences which make me think, is the ford variant special, better than the other 2?

5) The motul and valvoline say API: GL-4, GL-5, are those retro-compatible like Ford's specification on the engine oil FORD WSS-M2C913-C recomended but i pour FORD WSS-M2C913-D which is superior? (Or do these GL-(X) things even matter?)

I gotta say that the car (now little over 200k km, we barely drive it) shifts fine, in like 1+year of ownership we could not shift into 1st gear like 3-5 times? (maybe the clutch could have been not fully pressed on or something, we just put it in neutral and back in 1st and works, it's a very rare happening)

Thank you.

I asked my ford dealership when gearbox oil requires changing or topping up, 
their answer to me was...unless there's a leak then there's no need to try and top
up, the oil is designed to last the life of the car so it doesn't need renewing.

On another note this is about our Fiesta which is a ten plate, after about 8 years
into it's life changing gear became terrible, hard to push the gear lever into any
gears, I asked around and was told that the gear linkages would be all rusted up
these linkages all hidden inside a plastic black cover, on removing the cover they
was right! Everything was rusted up, so got to work with the WD40, to free up the
nearly seized parts, after everything was free & working smooth, I greased everything
and put the cover back on.

As long as it meets specification it's the same oil in a different bottle.

MTX75 is quite a strong gearbox, I wouldn't worry about it failing like the iB5.