If you have all the necessary tools and equipment to do the conversion, then the short answer is you will need 6.0 ounces of PAG 150 oil for a 1991-1993 A/C system.
Now the long winded answer. . .
If you haven't read the sticky
Basic Principals of Auto AC do so. It has a plethora of knowledge and information. I posted a link in that thread that provides a guide as to how much of, and what kind of, refrigerant and oil to use. Look at page 5 of that linked
.pdf to find the values for a Buick Roadmaster.
The compressor you should have is a GM R4 model and you will need to use PAG 150 oil. The amount of oil required for a 91-93 Buick Roadmaster A/C system is 6.0 oz. This stated value should be the same as what is mentioned in your FMS. However, you will be using a different kind of oil than the stated Mineral oil. Refer to the 94-96 Buick Roadmaster A/C system values to find the type of oil, PAG 150, which is compatible with the refrigerant R-134a.
As to the amount of refrigerant to add; with R-134a you should start with 70% of what is required then slowly add more to the system until you hit your desired air vent temperature. The linked
.pdf states that the system requires 50 oz of R-12 so. . . I would start with 35 oz (50.0 X 0.70 = 35.0) of R-134a and slowly add more until I reach my desired vent temperature of 45 degrees. Note the cars air conditioning system should be set for maximum A/C (60 degrees on auto climate systems), and the blower set to maximum too. You should have the car idling at approximately 1500 RPM's as well.
I will mention that you need to replace the low pressure cut off switch that is located on the accumulator/dryer. You need one that's for R-134a not R-12. You could try to adjust the old one manually; however I could never get it to work properly. It's a $20 part and no headache trying to adjust it. Also don't forget to replace the accumulator and orifice tube while your at it. If your compressor is eating itself you should thoroughly flush the system too. Use a good quality flush solvent and blow out each A/C component for a long period of time. Check the old orifice tube for metal flakes when you remove it. Make sure there are no holes in the screens, if there are, then chances are you have metal flakes in the rest of the A/C system. If that's the case I would recommend replacing the condenser too. No mater how hard you flush the condenser you probably won't get all the metal flakes out of it due to the design of the coils. You should replace all the O-rings with new ones.
At the very minimum you will need an A/C vacuum pump, and gauges to do the job. Have fun. I had to replace the compressor, accumulator/dryer, O-rings, Low pressure switch, and orifice tube last year. This was after taking it to a mechanic who converted it from R-12 to R134a and didn't use the correct oil for the conversion, if he used any at all. Yeah the compressor chewed itself up after a couple of weeks of use. You know what they say. . . If you want something done right, do it yourself. It's been over a year and no problems yet.
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