If the A/C gas has escaped the A/C system quickly, either from undoing the A/C pipes without firstly degassing the system, or from a front end crash where the condenser or its pipes gets damaged etc, then this can cause some of the A/C compressor oil to be lost along with the escaping gas,this is because some compressor oil circulates the system along with the gas, so if your car lost its A/C gas quickly, then its probably worth getting the A/C service guy to check the A/C compressor oil as well as just adding new gas. Its also worth getting the leak detection dye added with the gas so it will be easlier to detect any future leaks in the A/C system.. Also once the A/C is up and working again, be sure to use it for 15 minutes every week, especially in the winter time when it gets little use.This keeps the A/C system in good working order and keeps the compressor seals etc lubricated. Its also good to use the A/C with the air temp dial in the HOT and the position set to screen to demist the interior of the glass heaps quickly, as the A/C system also dries the air as well as cooling it.