Silicon212, why the ester oil instead of the Pag? GM recommends the Pag......
Also it's recommended that the mineral oil be removed(and measured) from the compressor, and replaced with Pag......and then enough Pag oil should be added to the system.....typically 8 oz., total.....the rest of the mineral oil that is within the system, is compatible with the Pag.......like you say, the accumulator(with new desiccant) should be replaced......naturally, it would be better if the system was flushed of as much mineral oil as possible....but you will never get rid of all of it from the evaporator and condenser......
DO NOT USE CHEAP RETRO KITS! The valves will leak..........
formula for 134a is: multiply recommended r12 weight by 0.9......then from that, subtract 0.25 for the weight of r134a.......
for example......if the capacity of your r12 system is 2.0 pounds, 2 X 0.9= 1.8......then subtract 0.25 from that and you get, 1.55 pounds of 134a.....
The reason for this is to decrease head pressures.....but who has a sensitive enough scale for this? Only the expensive a/c machines do....
I would, personally, fill by pressure, using the appropriate graph of ambient temp and humidity.....
Unfortunately, most people don't have vac pumps to pull a vacuum on a system to do this job....I personally would let a shop do it......shop around....