ATF that would meet Dexron III specs would be suitable. some older Gleaners called for Dexron (like the first Dexron made, aka Dexron I )
The high heat resistance, aggressive detergents, cold weather flow, etc. are reasons why Gleaner made their choice.
I imagine for a summer time only combine, some good 821XL would be suitable. If you want to differ from factory specs, I would contact a reputable hydraulics outfit, tell them what pump and hydro you are running, and get their opinion.
Like said above, the newer ones went to hyd. oil.