What is AC oil charge?
Hereof, do I need to add oil when recharging AC?
You normally do this when recharging the system after replacing one of the major components. This is tricky. The service manual normally tells you how much refrigerant oil should be added with each major component replaced on the AC system. Off-the-shelf cans sometimes come with refrigerant oil in them as well.
Keeping this in view, what is AC oil?
Like any mechanical system, it's important to make sure your air conditioning system stays lubricated, and AC compressor oil is the lubricant for the job. Your AC compressor creates a low pressure to draw refrigerant into a chamber.
When air enters a refrigeration system, it collects in the top of the condenser and is trapped. Air will cause a reduction in condenser surface area, which will cause a high condensing pressure. Air can enter the refrigeration system through a leak in the low side of the refrigeration system.