Power Steering fluid

Okay, getting my car ready to start and I need to fill up the power steering pump, as I had it off to paint. Anyway, it says on the cap, "Use transmission fluid Type A". I went to the auto parts store and they looked at me like I was nuts. So, what is the modern day equivalent? Thanks.
 
My Walmart and Napa both stock it. Its fair in price compared to Merc III. Type A is also used in older hydraulic (power)top and window assemblies and as shock absorber fluid in my 1937-63 Packards, Kaisers and Fords
 
In the 1950's & 1960's, GM used hydraulic (brake) fluid in the top mechanism.. Don't know about Fords...
 
So I can just use regular modern day power steering fluid? Can it be mixed with what is already in the system (I didn't drain the whole thing, just lost some when I took off the pump). Sorry, the transmission fluid recommendation on the cap threw me.....
 
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