The original system will work with the retro fitted refrigerants. You don't need to change the oil in the compressor. If the system has been open to the atmosphere for any length of time I would recommend changing the filter drier/strainer. Once you bolt and tighten everything up you will need to put the system into a vacuum. (29.9 inches of mercury). This allows all water vapor to boil off. It can take as little as 15 minutes, or as much as an entire day to rid the system of moisture. It all depends. A reliable set of gauges and an electronic thermistor gauge makes it a precise science. However, you can get by with just a set of gauges. Once the vacumm has been achieved, allow the system to sit to check that the vacuum is holding. This will prove the system is leak free. Then charge the unit by weight. Most compressors have a tag that states how many ounces of refrigerant are required. It can be off a tad so you don't have to sweat the detail. A thermometer in one of the duct openings in the cabin will tell you when you are putting out the correct temperatures. NOTE: Car AC systems are grossly oversized in order to cool down what for all practical purposes is a glass bubble with an insulation free metal roof. Remember, the retro fit refrigerants are intended to be used in conjunction with R-12 systems. The gas you charge the system with is mixed with oil in the can.