Someone gave you bad information. The system has no sump, the
refrigerant oil circulates with the refrigerant and so flows throughout
The initial oil fill for a R-4 Frigidaire compressor is 5.75 fl oz LESS
what's already in the system - in the evaporator, in the lines,
condenser etc. Your guess as to that residual quantity is as good as
Once you open the system the receiver-dryer should be replaced.
I bought an oil cartridge for R-12 systems that I attached to the
suction fill connection. The cartridge has a screw that one turns to
force the oil into the running system. The oil quantity is known so one
doesn't over do it and create a hydraulic lock inside the compressor.
Each of these oil cartridges cost about $10 and avoided opening my old
car's R-12 system.