Good evening, the ideal is to use a direct substitute for R12, such as R401A (MP39) or R401B (MP66), you would only have to change the dehydrator filter without changing the capillary, only there are countries in which these refrigerants do not They are for sale, an example is here in Venezuela, where obligatorily to put into operation old refrigerators that use R12 as a refrigerant, we must replace this refrigerant with R134a, even though it is not recommended since R134a is not a substitute for R12 However, they have worked excellent, what should be done in these cases is a deep cleaning of the entire system since the mineral oil that comes with the compressors that work with R12 is not compatible with the R134a, therefore, Once the internal cleaning has been carried out, the compressor oil must be replaced with synthetic Polyolester oil (POE), up to a viscosity of 32 is recommended for low temperature applications (refrigerators d ), the dehydrator filter should be replaced and, if possible, it is advisable to replace the capillary tube according to the refrigerant capacity in btu / h of the compressor, however, personally, I had to do this work in refrigerators where it is very difficult to replace the capillary tube, I have left them original and the refrigerators have worked very well, but whenever possible, it is advisable to replace the capillary tube.