The oil is not mineral oil. You can purchase a recharging kit that has the oil incorporated in the can with the refrigerant. You will need to purchase a conversion kit as R-12 is no longer available and you will need to put R-134 in the system. Secondly, if the refrigerant is low then the compressor will not kick in when the AC or defroster is turned on so don't worry about adding the oil until you recharge the system.
DON'T USE MINERAL OIL AS IT WILL RUIN THE ENTIRE SYSTEM.