Old rules of thumb state that you should add so many ounces of oil when you replace a condenser, so many ounces when you replace an evaporator, etc. But your air conditioning system demands more precision than that.
Oil in an A/C system has one job: to lubricate the compressor. Most replacement compressors come pre-charged with 5 to 10 ounces of oil depending on the make, model, and application, but some take less and others require more. The type and amount of pre-charged oil should be clearly marked on the compressor’s label (a more accurate source than product literature and catalogs).
The pre-charge means there should be no need to add oil when you install a new compressor.
When you have too much oil in the system, it can coat the inner tube walls of the evaporator and condenser and diminish the ability of the coil (evaporator and condenser) to absorb or dissipate heat. Because excess oil can collect at low points in the suction line, you also risk slugging the oil into the compressor at start-up.
There’s only one way to know how much oil to put back into the system. You should measure what you’re taking out.
When you’re replacing major components, drain the oil from the old component and add that amount back into the system. If you’re replacing a compressor and not flushing the system, drain the old compressor, measure the amount you’ve removed, and then extract the difference from the new compressor before you install it.
Today’s refrigerant recovery and recycling machines can separate the refrigerant and oil and measure the amount recovered. They can tell you the condition of the oil and approximately how much new oil to be added back into the system.
Still, there are no guarantees. If a driver or mechanic looking for a quick fix decides to shoot a 1-pound can of refrigerant into the system, he may be injecting a little oil and/or sealant as well. When the vehicle comes in for service, there’s already more oil in the system than there should be. You can replace what you recover and yet end up overcharging the system with oil.
But it’s an important concept to grasp. When you add oil to a system, your “best guess” is never as good as a precise measurement.