That "Parts Store" advice is 20-30 yrs. out of date.....
It's been about that long since all manual transmissions used gear oil...
SO, the "Right" lubricant will make the trans work easier/last longer than a "generic" fluid - the "wrong" thing prob. won't "break" it, but you won't be happy with the results!
For example, imagine how stiff the shifting would be if you used gear oil in a trans. that was originally filled with ATF on a cold winter's day!!!
The Ford synthetic fluid is quite pricey, but when using it the trans shifts "slick" and you won't have to worry about excessive wear since it's the best lube avail. for the purpose.