Curious where you downloaded that info because it's inaccurate. Most hydraulic oils below AW-46 do not carry an AGMA rating and are not indicated for geared drives because they aren't formulated for it. Or maybe you are misconstruing the use of hydraulic oil with Universal Tractor Fluid (UTF) which in some cases it is used in the transmission, differential, and hydraulic system on some OTHER tractor brands.
And to expound on Rich's post a little: Engine oils generally make poor gear lubricants mainly because of their additive packages and the lack thereof. There are some gear lubricants that are formulated for extreme shear like hypoid and worm drive sets where there is a lot of sliding tooth contact not found in spur gear configurations. Still, you need a lubricant formulated for the task. It's actually very simple: Engine oil for engines, gear oil for gears, and hydraulic oil for hydraulics.
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