My '67 Armstrong A/C unit is an R-22 unit. It does not care what the outside temperature is. Although I have paperwork on what the right pressures are, when I had Johnny Mosely replace the compressor, he humored me by casually looking at my paperwork and then explained that the pressures are a function of the ambient temp, and the experience of the installer.
I kept my mouth shut until the new compressor was installed and the air conditioner was all but turning my Overlander into a meat locker. Then I asked Johnny what the pressures were. He got pretty smug when I told him he had nailed what my antique paperwork called for.
The complete tale is at my Overlander's web site.