In the past, many A/C shops had a conversion book that showed the recommended R134a charge for a conversion from R12, but those books are probably long gone in newer shops. Older conversions also still used mineral oil (IIRC) with R134a, but now recommend cleaning out the system and switching to PAG. I also switched to a Ford Blue orifice instead of the GM white orifice, because its supposed to work better with R134a.
The Pro6Ten compressor is supposed to be a new compressor in the same form factor as an A6, so it can be used as a direct replacement, but I've never actually seen one. Older rebuilt iron A6's were so unreliable that they coined the term "black death" to describe how they deteriorated and failed internally. It sounds like maybe the parts numbers above are a newer A6 model that is more reliable. After my first rebuilt A6 failed I paid for a shop to make new barrier hoses, and switched to a Sanden and brackets from Vintage Air. Its a lot smaller form factor.
As you make the conversion, if you have a 2 groove water pump pulley, take a look at Rock Auto and see what options you have for PS belts. My truck has the HD cooling option, and you can use a longer PS belt that goes over the crankshaft, water pump and PS pump. This means you have 2 pulleys driving the water pump, and if you lose one of them you will still have an operating cooling system.
Bruce
R134a is not miscible with mineral oil, so that was a recipe for disaster. The aftermarket always recommended POE oils for conversions, but interestingly, no OEM would guarantee compressors when POE was used.
The term 'black death' was more often used with the Ford FS10 compressor. The Teflon coating on the pistons slowly sloughed off, leaving the most awful goo through out the system. It has come to be thrown around for lots of A/C failures, but it had a very specific origin. It was not easy to clean up either.
I'm surprised to hear anyone claim an A6 was unreliable. They will stay lubricated and running even with an empty system (also like the old, single cylinder York/Tecumseh and RV2). They were so far ahead of their time (the NVH of the York/Tecumseh and to a lesser extent the RV2 were awful in comparison), and so superior in operation and durability, they were not only used by GM exclusively from 1962-1975, but also in Mercedes Benz, Rolls Royce, Jaguar, many heavy equipment manufacturers and some Fords.
Like so many things, GM threw out the baby with the bath water with the R4 compressor (never branded as Frigidaire, but as Delco Air, Harrison and finally Delphi). I'm still shocked Delphi bragged about the Scotch Yoke design on their website. That it is why they are so noisy!
I was intrigued by the Pro6Ten compressor when it appeared. I haven't any experience with them, but would suspect they are good. With the supply of rebuild-able A6s (I wouldn't buy
any rebuilt compressor) drying up (forget what year they were last built, sometime in the 80s, IIRC), there was apparently a need, as so many were fitted in vehicles and equipment originally. Besides the labor (and skill) to replace the front seal in a functioning A6 is cost prohibitive.