What would you consider expensive?
Replacement of transmission due to wrong fluid or just the right fluid but more $$$ to put it in.
Ok, not to be too redundant but if you have a lot of miles on it, then changing the fluid is the better option than to do nothing.
Here's the rub...
Whether it's an iB5 or any typical B5/B6 or even typical synchronized 5-speed - they all need gear lube, but have requirements to that...
The older Ford types used to use Ford Type-F - not anymore - they use the GL-4 or GL-5 specification but there's a catch.
The older 6 speed without that special transmission and clutch design they called Powershift - which you select the gear but it used a series of solenoids and actuated the clutch (two really) to shift into the selected gear - electronically. Needed a lube type that was specific Was part of a lawsuit amidst of the complaints.
So the above list, I'd look for the GL-4 spec, why? Because the GL-5 is for limited slip differentials and the friction modifiers may not settle well in being used in an older worn transmission with uncertain unknown conditions pertaining to the gearbox's integrity. GL-4 was for earlier versions - GL-5 was for the newer crossovers and other drivetrains that were for AWD - which have specific applications and coatings on their meshings and geartrain.