ATF vs 821XL?

Does anybody know why Allis specified automatic transmission fluid in the hydraulic and hydrostatic drive systems of combines instead of just using regular 821 hydraulic fluid? Would it be detrimental to use hydraulic fluid instead of automatic transmission fluid? I figure there has to be some reason they did it.

Hydrostatic combine transmissions existed before 821XL was used in A-C machinery.   I call 821XL a 10W-30 viscosity and the ATF fluid a 5W or 10W viscosity. Gleaner did switch to the 821XL about 20 or so years ago but there were orifice changes made in some of the hydraulic valves and I'm sure the hydrostatic propulsion system must have some changes too. Make the change at your own risk.

Somewhere around 40-50 years ago usually in late summer our dealer would invite all combine owners in and would have a new combine sitting in the shop. There would be a AC guy there (I think they called him a "Block Man"). He would be the guy the dealer called in if there was a mechanical problem they couldn't solve. He would walk us around the combine and point out things to look for so that we could reduce the odds of having problems during harvest.

Lynn remembers it well !!   The Sales Rep for A-C had a "block" of dealers he was responsible for. He ordered new machinery (and attachments) for his dealers and kept track of inventory to see that something didn't disappear. I doubt that any blockman ever had more than 10 businesses he called on. The Service Reps had more like 25 to 30 dealers they each called on. For the whole state of Iowa there were only four service reps, whereas the sales force had eight or ten people. I remember a true story about a blockman who was taking a particular dealer's inventory from looking out the office window all the time. There was a particular 200 tractor that wasn't in his line of sight from the dealerships window. He asked where it was and was told it was out beyond the shop, and he could drive his car around to see it when he was leaving. Well, he was never fired over this deal, but the story was the tractor had been stripped of many parts. The business was in financial trouble and was on C.O.D. for their parts inventory, so the starter, batteries, tie rods, and on and on were taken from this tractor and sold over the counter to customers. The blockman didn't catch this and months later the business filed for bankruptcy. The 200 tractor was missing many more parts as time went on !!   This is why AGCO and Deere and all the others love the multiple stores model of business. They have less dealers to work with, so it takes less blockmen to cover them.