Consider that R134A is going away. I use Duracool hydrocarbon refrigerant, but not pushing it on others since many non-technical people make absurd claims about it being dangerous. HC's are a direct swap, i.e. can work w/ your existing mineral oil.

I would never use stop leaks since all of the ones I know of work by putting aggressive solvents in your system which are supposed to swell old rubber O-rings so they seal better. I imagine other things like hoses swell and degrade as well. It isn't terribly hard to change all the O-rings (use HBNR), except the shaft seal on the compressor.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans