The AC compressor is either on or off, thereís no in-between, and is controlled by a pressure switch in the low pressure line (way back on the firewall in the S197, which makes it hard to get to). When the compressor kicks on the needle on a low side gauge will go down, and when it kicks off, the needle will float back up. Hard to say how much refrigerant the system actually needs, about Ĺ to 1 of the R-134a service cans is typical (mine took a whole can to get it ice cold again).
Too much oil can be bad for a system, fluids arenít compressible, plus the oil takes up space that could be used by refrigerant, and unless you have a serious leak to where youíve lost oil, it shouldnít be needed. Better to just get a can of straight R-134a in your case (without the oil). A slow loss of refrigerant in Ford AC systems is a fairly common thing, unfortunately.