Oil in a AC compressor is there so that the mechanical parts don't over heat from friction
To prevent wear on the compressor.
The compressor oil
Oil
Oil in the compressor may leak into the refrigeration system, this oil must be allowed to settle back into the compressor, or it may cause a breakdown due to system blockage
Sludge in refrigeration is a combination of water, acid and oil, normally this combination will eat the varnish on the windings of the compressor and short it out.
I'm in the process studying for my epa test and this is one of the questions and their answer is foaming occurs in the compressor.
To return oil to the compressor.
to lubricate the compressor
mineral oil
Did you add an oil charge to the system when you recharged it and did you recharge the system using refrigeration gauges. The system may be low on refrigerant oil or you could have overcharged the system causing the compressor to have to work too hard.
freon and oil
It means charging the A/C system with the proper kind and amount of refrigerant + oil. Overcharging the system will damage the compressor.