400 hot hydraulic fluid

Farmall 300 and 400 tractor hydraulic systems ran very hot even back 50 years ago when they were new, especially on loader work or other high demand hydraulic function. If your happened to touch your arm on the lines from pump to controls, it would burn your arm. Have been there and done that one.

Here is the problem and solution. These systems have a very low oil capacity, don't remember exactly but 8 to 12 quarts. This works well if your using hitch only or a single implement cylinder in an operation like plowing. The solution is add resovoir. IH dealer added a 5 gallon resovoir to my dad's 300 when had problems on loader work. This is the history of row crop tractors in general, they built loaders for them, but they weren't engineered for it.

Don't let this deter you from what Nat has said, excessively hot may also be problem as well. If you wish to try adding resovoir, its quite simple and inexpensive to do.

By "hot" do you mean excessively hot, or just operating temperature hot???

Is this a "factory" power steering or an add-on? Is it powered by its own pump, or the tractor's main hydraulics?

Excessively hot means there's a restriction in the system causing the pump to work extra-hard when no hydraulics are being used. There's a screen on the side of the steering pedestal that may need to be cleaned. Then again, one of the valves could be stuck partially closed. Basically, you need to go through your hydraulic system component by component, work it over, to find the problem.

Operating temperature hot means something in the system is badly worn, either the pump or the motor.