[Bigdav160]

Back in the day of R134a retrofits, GM didn't recognize the need for flushing. Mineral oil and PAG do not co-mingle. In the presence of R134a just settles to the bottom of the system. GM doesn't recommend ester oil either. I do know many people use it as it is cheaper than PAG.

To the OP, your compressor is fine with R134a. If those fittings fit the compressor and the hoses, then they will work fine. As mentioned, the POA should be recalibrated. That is not a job just any A/C tech can do. F.A.Benny, on this board, can do it for you.