Richard is offering very good advice. Stay with R12.

R12 is only slightly more expensive than any other refrigerant. Any expense you incur to change to another refrigerant will gobble up any savings that you might enjoy due to less expensive refrigerant.

Check for voltage at the clutch connector going to the compressor. It could be that the clutch itself is bad or needs adjustment.

The pressures you are reading are proper static pressures. Whether it has a few ounces or is full to the brim, those are the pressures to expect without the clutch engaged.