We once had some FG25ST-4s and FG25ST-8s in our company fleet (they have all since been sold).
The service manuals we had for those trucks specified Dexron II.
With the advent of Dexron III in the early 1990s we began using Dexron III in the fleet. Dexron III is "backward compatible" with Dexron II.
While the clutch discs cannot be replaced without removing the transmission from the chassis, the clutch discs can be somewhat inspected by removing the control valve from the transmission case.
Once the control valve is removed you will be able to see the edges of the discs through the spaces of the clutch drum.
The metal clutch plates are mated to the clutch drum with "drive tangs" on their outside (circumference) and these discs are all steel.
The friction discs are mated to the clutch pack center hub with "splines" and have friction fiber media on their faces.
You can look at the friction discs edges and tell whether there is any friction media remaining on their faces by noting whether there is a very slight spacing between each friction disc and its adjacent clutch plate disc. If the friction disc METAL SURFACE is completely against the clutch plate disc.........there is no friction material remaining. You may also pry the discs apart from each other with a flat blade screwdriver to get a better view.
Another thing you must check for on the friction discs is to note how these discs are "resting" in relation to all the other friction discs in the pack. If any of these discs seem to be lower on their edges than other discs......the internal splines are likely "shelled off" or beginning to.
You also can place the edge of a flat blade screwdriver against the circumference edge of each friction disc and try to rotate the disc. If you are able to turn the disc, it has shelled off all of its internal splines. You should not be able to turn a friction disc in this manner and when a single friction disc moves, it should move ALL the other friction discs in unison.