I can understand the pump manual saying use whatever the OEM recommends. There may be other considerations like wet brakes. When you're dealing with just a couple of differential gears that need to be lubricated, ATF should do the job. But I'll go along with the manufacturer recommendation.
The manual on my 1450 says any fluid that meets IH B-6 specifications will do, and then goes on to say for maximum protection use IH Hy-Tran fluids. IH B-6 was replaced by MS-1207(Hy-Tran Plus) and then that was replaced by MS-1209 (Hy-Tran Ultra).
There are many fluids that meets these specifications on the market and are therefore suitable for use per the manufacturer recommendation. Even the Super Tech hydraulic fluid found at Walmart meets these specifications.
The only thing that remains to justify paying the higher cost is the water absorption capabilities of Hy-Tran. The claim is that it can absorb 1% of its volume in water. A cub takes about 2 gallons, and 1% of that is 2.5oz, or 3/8 of a cup. It's a personal decision whether the 3/8 of a cup of water is worth paying 2-3 more for the oil. To me it's not.
