There's always compressor oil of a specified amount within the refrigerant system, don't matter whether its your home fridge, room aircon or even car aircon. The common conventional domestic refrigeration appliances and car air-conditioning all depend on compressor oil within for lubricating the compressor in operation. Therefore, whenever there's refrigerant leakage or system is evacuated, there's surely some oil loss in the process, not all. Its therefore required to add some (only a few cc) oil to make up for the loss. In event of a total stripdown of the compressor, the installer have to fill specified amount of the right type of oil, vacuum and recharge the system.
The compressor would eventually wear down, become noisy or fail if there's no oil in the system.

If you choose not to maintain the compressor, you can choose to replace the compressor when time comes.

Is true when aircon is on, car is underpower.

It also depends what is your car's C.C.