There are several types of oils on the market for AC systems. If you add oil for any reason, you CANNOT mix them. They react and can cause major issues. I have read some reactions will eat the system up from the inside out, others create solids that plug up the system.
It probably says something like POE, PAO, PAG oil or some other combination of letters. If you can post the pic of the yellow sticker someone will recognize the oil type.