The only car in my fleet which uses r-134a is the 300TD; it was converted already when I bought it. The sticker says they used ester oil and 28oz of refrigerant. I hooked up my gauges to the low-side port by the airbox and it read 0psi. I proceeded to add 12oz of 134a, checked the pressure (all this while the car was idling and the AC blowing) and the pressure was 22psi. I added another can of 12oz of 134a and after that the pressure read about 34 psi. I turned off the AC and the gauge reading went up to 55-60 psi. The whole time, the AC was blowing hot air. I turned the car off to let everything cool off and equilibrate, and I'm about to go out now and check it again.

Concerning the oil, when I retrofit our other cars, I will evacuate the whole systems and use PAG oil - I only kept the ester in the 300td because it was already converted and it was working ok when I bought it.

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'82 300TD
'77 6.9
'75 280S
'74 280
'87 Porsche 944 turbo