Engine oil contains additives to do things for your internal combustion process engine. Other parts of the tractor don't have internal combustion so they don't "require" the additives.
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Most tractors of today run with a mineral oil of around SAE 20 wt in all places but the engine. It is aftermarketed most everywhere (wally has it) as "Trans/hydraulic" fluid or some other associated trade name; OEM specks are on the back. The engine gets a high detergent oil especially formulated for that purpose; hence "motor oil".
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What I think I hear you saying is that the tractor runs everything with one oil. Since you have to have "motor oil" in the engine, sounds like they put motor oil in everything, which is ok if you keep the viscosity you need. Considering the viscosity that things like power steering and hydraulics need it sounds like you have a 10w-30 viscosity high detergent motor oil to run everything. So if it's gas you can go and get a 10w-30 API rating of SJ motor oil, or if diesel, a 10w-30 API rating of CI-4/SJ, or thereabouts. (The rating is on the back of the container inside a little star). I think I gave you the current characters for ratings. They keep changing all the time as EPA requirements change. You can at least get close to these figures.
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Now this is a SWAG but not that wild of a guess. Having dealt in these sorts of things for quite awhile that's how it looks to me. I do know that motor oil is used elsewhere in some equipment of this type so I don't feel bad about what I think is your case. If that doesn't suit you, go contact the mfgr and see if you can coax him into telling you what the generic name of the liquid he uses happens to be. Good luck, Mark
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