The late 123 300D's had GM R4 compressors.

Evacuating the refrigeration system every three years and adding 4 ounces of oil every time will ultimately lead to a severe oil excess. Severe oil excess will lead to a severely locked up compressor.

Flushing the entire system is not too big of a deal as long as you are equipped for it. It means you will need a good vacuum pump (preferably in a recovery/recycle machine,) manifold gauges, a flush gun and cannister, flush agent and a good air compressor.

If you have the proper equipment, simply recover any refrigerant and break all connections and use your flush gun and cannister to push flushing solvent through each component individually and then MAKE SURE that ALL flushing agent is blown out of every component. With the ENTIRE system flushed with solvent EXCEPT the compressor which is flushed with refrigerant oil, put the correct amount of oil in the system which I think is 8 ounces. Pour an ounce or two in the compressor if it will fit and then of the remaining 8 ounces put half in the evaporator and half in the condensor.

Connect everything back up with new o-rings, putting a new receiver/dryer in place LAST. Evacuate and charge the system.

Good luck,