One of the most confusing parts of auto A/C service is understanding how much oil to add and if oil should be added to the system. Once that is determined, the next question is where to add the oil. Before adding any refrigerant oil the A/C system, consider that the A/C system is a closed loop system. Oil should only be added to the system when replacing parts or if you have determined that there has been a loss in oil.
The amateur A/C service tech will add oil every time the system is charged. That is probably because the equipment salesman told him to, or the salesman sold him a fancy oil injector. Adding oil when it is not required can lead to an oil overcharge, poor cooling performance of the A/C system or even, in some cases, a seized compressor.
The average A/C system holds between 8 and 12 ounces of A/C compressor oil or refrigerant oil. The only reason the oil is in the system is to lubricate the compressor. There is no other components in the A/C system that require oil. The oil travels as a mist throughout the system. The compressor requires that mist of refrigerant and oil because A/C compressors do not include any oil storage or oil sump.
Typically, oil is added when components are being replaced, and specially when the compressor is being replaced and the A/C system is flushed. The general rule for adding oil to components is as follows:
Receiver drier - 1 oz.
Accumulator - 2 oz.
Condenser - 1 oz.
Evaporator - 3 oz.
Hoses after flushing - 1 oz.
Compressor - Drain old compressor by turning the compressor with ports at the lowest position and turning the clutch to move internal parts. Add the same amount of oil as removed plus one ounce.
Whenever adding oil, remember that it’s there for the A/C compressor only. Therefore, be sure there is always some oil in the compressor. The balance of the oil can be added into any components. Ideally, the oil should be added upstream of the compressor. That is, in the accumulator or the evaporator. Once the A/C system operates, the oil will be distributed and seek its proper level through the A/C system.
If adding oil because you suspect a loss such as through shaft seal leaks, understand that one ounce of oil sprayed under pressure and leave a large oil stain. Use caution so the A/C system does not become overcharged with oil.
The problem with adding too much oil is that excess oil causes oil slugging. That is the formation of a film of oil internally on the heat exchangers of the A/C system. Those heat exchangers are the condenser and the evaporator. When the heat exchangers develope an internal coating of oil, the oil acts as an insulator and restricts heat transfer. When that happens, cooling performance is lost and pressure gauges will typically show higher than normal pressures.