I believe the system takes 9 ounces of PAG150 oil. and realistically the type and viscosity of oil is determined by the compressor manufacturer so if you were to replace your compressor you would want to double check with what that compressor manuf. calls for in case it's different than what GM used.
General practice is if you replace an ac part such as a compressor that you pour out the oil from the part and measure it then add that much fresh oil to the new compressor and install. Oil is distributed throughout the system so you want to try to maintain 9 ounces of oil in the system. For an accumulator you drain and measure the oil then add that much plus 2 ounces because the accumulator will hold approx 2 ounces. For just an ac line i would not add more an than ounce considering the coating of oil inside the line. If it's a long line then consider a little more, or if oil drains out then measure and make a best guess. the alternative is since the system is opened is to flush everything then add 9 ounces of oil. like anything else clean and fresh is best so if you can wrench on it then consider pulling the compressor, draining & refilling with fresh oil. if it's a year 2000 replace the accumulator. And use mineral oil on the o-rings on the lines, not pag oil. pag oil will deteriorate the o-rings, and don't get mineral oil into the system. last thing you want to do is put money and time into servicing it then have a problem.
try calling local shops and see if they will work with you on ac service. if you are comfortable wrenching yourself then you can do most of the labor and then bring the car for a vacuum and recharge. ask if they have the machine and if they can inject oil when they charge it or if you should add the oil beforehand.