Shaft seals on AC Compressors are not lip seals like you would see on a low pressure seal like a main seal. They are a type of seal that looks a lot like the seamless ring seals in a transmission.

They should be R134a resistant because they are not rubber. Side note: Never handle a compressor seal with your hands. According the manufacturers, it ruins them.

I have converted a lot of systems over to R134a and I have not had a leak at the compressor shaft seal.

All have leaked down over time much faster than an R12 system would. It may be the smaller molecules escaping through the hoses not designed for R134a or the fact that R134a tends to run higher pressures and the result is small leakage.

Mineral oil was used in R12 systems. Mineral oil will not carry in R134a so it settles in the bottoms of the compressor, drier, and cores. So you will need to change the oil or at least run Ester oil. PAG and Mineral oil are not compatible together.

Fill your system by pressure not the quantity, books will tell you 80% R134 to what the original R12 system used but that is usually too much. Pressures are temperature dependent.

All that said I have no problem using the old system if I want it to look stock but it will never cool as good as a properly functioning R12 system.

If you want the absolute best using R134a gas, then install a matched component system made for R134a.

My