Most people use the Type 55 hydraulic oil that Oliver recommended, but that is getting scarcer to find. I used to run a 10W hydraulic fluid, but decided to switch to universal hydraulic fluid to prevent cross contamination between tractors, plus fewer oil types to keep around. Universal is a little heavier, so I give a little more idle time to warm it up in cold weather. I have been told not to use universal in the older hydraulic systems by more than one person, but AGCO now recommends it. I have been doing it for several years with no ill effects. If you go with a 10W, try to find a hydraulic fluid instead of motor oil, because of the anti-foaming agents.

I was thinking that the manual steering gear box was supposed to be filled with grease, not oil. If I were to put oil in, I'd use 80W-90.

For the power steering, Oliver used ATF in units that had their own pump. I personally use the automatic transmission fluid (ATF). You won't be able to find type A any more, but a Dexron or Mercron fluid will be it's replacement. Basically, if it is red ATF, it is fine with the exception of type F. This was used by Ford, and I have been told it has friction additives that you wouldn't want in a regular hydraulic system.