Power steering pump

So the pump is just a low pressure gear pump. It will work with engine oil or atf. The atf has better clutch plate wear properties and a lower pour point.

 

Should be able to use either or. I would stick with whatever is currently in the system to avoid mixing fluids. You'll never get 100% of the old oil out unless you completely tear down the steering box.

 

It really went by the steering sector mfg recommendation.
Sheppard used 15 W 40.
Ross - TRW used ATF.
A major I worked for had new trucks come in with Ross boxes. Ross said they would not honer warranty if ATF was not used. Company said we have over 300 shops and have no reason to inventory ATF. Even the service trucks are manuals. If we have to add all the inventory for your steering boxes we will make a point not to buy new trucks with them anymore. Then 15 W 40 was fine.
The 2019 Pete here with a Sheppard is labeled for ATF.
Your concern is the warranty of your new pump. Flush the system replace the filter and use the fluid they specify.
An exception to that could be a Saginaw or ? ( some other unusual sector ) . They can have a be reaction to anything but P/S fluid.

 

Name is normally cast in the side of the box or post a picture.