1991/C1500/5.0L,305: I'm replacing the A/C compressor (R4, previously retrofitted to R-134a) and need to know the recommended refrigerant oil and capacity. The parts store gave me PAG 100 but the little bit of info I could get from research online indicates it's more likely PAG 150 (which the parts store didn't have in stock). Will it make a big difference which is used and how much should go in the new (remanufactured) compressor?
If you are changing the compressor because it blew you should seriously consider flushing the system with an approved A/C flush to remove all contaminants. Installing an inline filter wouldn't be a bad idea either to protect the new compressor from any contaminants left over. Metal from the bad compressor is all over the system. Some parts can't be flushed because of their design like the accumulator. Sometimes replacement of the accumulator is necessary. Remember there are no short cuts for A/C repair. If you are not sure of the repair this is a repair for a professional.
I'm not sure of the viscosity of the refrigerant oil but it shouldn't make a difference the PAG 100 is widely used. If you want to play it safe you can buy the oil at a GM dealer, the part number is 12345923 and the amount of oil to fill for that system is 6 oz of oil.