nauman bhai,compressor oil is the life blood of an AC system,oil in whole AC system is only for the compressor,no other component require internal lubrication except AC compressor.
Whenever you go for servicing AC,extract maximum used oil from compressor like 3s guys extracted in proverbial_slang's case thats definitely right way.If your car's compressor starts malfunctioning due to less oil,compressor's internal parts may break and their microscopic debris may clog condenser's linings,expansion valve,orifice tube,accumulator or drier,so when you are going to replace a compressor you should have to completely vacuum the whole system including condenser,drier,accumulator and should have checked the orifice tube and expansion valve.
In most cases where mechanic directs you to replace the compressor,ask him to check thoroughly for any debris clogged inside the orifice tube or expansion valve which are common reasons to which they neglect and persuade you to spend on compressor.
Very important is to remove as much contaminated oil as possible while servicing.Its just like a new engine installed on you car ,you wont reuse the old oil.Do you?
