If you have already converted to R134A refrigerant, they have a mix of freon and oil in a can that you can use for your first refill. After that use the straight R134A to fill to the proper charge.
If you haven't converted from R12, you will have to replace it with Freeze12 or another substitute.
The system uses about 7 Oz of oil per total charge.
I would not add more tha a couple of ounces. If there is to much oil than there is not enough room for freon and the system will not work properly.You probably did not lose as much as you think but with the age and failure 2 ounces should be plenty just my 2 cents Steve