Long ass write-up: A somewhat comprehensive R12 to R134a conversion guide
Preface to this guide with quick relative information and some interesting tangents: If you've got an NA 1990-1993, they used R12 as the refrigerant for the AC system. This is an old school refrigerant which the EPA deemed bad for the environment, so they mandated the switch to R134a.
Side note: the EPA is phasing out R134a for R1234yf refrigerant in the newest cars (2020+), which is supposed to be better for the environment than R134a.
Super side note: There are "R12 compatible" alternative refrigerants available which are not R12, but instead a different gas altogether: Propane/Butane/Isobutane mix. Propane has very very similar latent heat (condensation and evaporation properties) as R12. Theoretically, and practically, you can charge an R12 system with Propane and it will perform just as well at refrigeration. Propane is also friendlier to the environment than a lot of dedicated refrigerants. Propane is commonly used in industrial refrigeration applications and is designated as R-290. However, it is illegal to charge automotive AC with propane, and many consider it unsafe due to its flammability and volatility.
(In my opinion, propane is kind of tricky to ignite without perfect stoichiometric mixture, and there's less than two pounds of refrigerant in the Miata's AC system, I don't think the safety concerns of running propane is anywhere near as concerning as the 10+ gallons, or 60+ pounds of gasoline fuel circulating between the fuel tank behind the driver and the engine in the front, but I digress...)
Anyway, R134a is still widely available and ubiquitous. R134a is technically less efficient than R12 as a refrigerant, but not by much in an automotive application, since target evaporative cooling temperatures in automotive applications are not to exceed temps cooler than 32°F. Both refrigerants are capable of reaching temperatures far below subzero, but with different expansion valves, compressors, and condenser layouts. Cooling temperatures of vapor cycle refrigeration systems are not to exceed temps lower than 32 degrees unless ice/freezing water is desirable, like in a dedicated freezer. That is to say if temperatures get colder than 32 degrees, the evaporator in your car can freeze up and prevent airflow through it, resulting in highly restricted airflow from the cabin vents.
Put simply, R134a is capable of running your converted AC R12 system at sufficiently cool temperatures for your Miata. You'll only see a difference of a few degrees warmer with R134a vs R12.
The things that must be changed to convert R12 to R134a are:
-Generic R12 to R134a service port adapter fittings. (Preferably right angle fittings for better clearance.) These are cheap and easily found on eBay or Amazon.
-24 oz of R134a refrigerant. Two 12 oz cans from the parts store.
-Parallel Flow Condenser There are two versions of refrigerant condensers for the NA Miata. Serpentine Flow, and Parallel Flow. The R12 system uses the Serpentine flow condenser and it is incompatible with the R134a refrigerant. R134a will struggle to condense, and it will not subcool without the proper parallel flow condenser. This will lead to very high pressures on the high side, stress the compressor, stress your engine with additional load, and trigger the high-side pressure safety switch to deactivate the AC compressor frequently. Basically, your AC won't even work if it's warm outside. The Serpentine flow and Parallel flow condensers are 100% interchangeable with each other in all NA's 1990-1997, so it's easy to overlook this part. You need to make sure the condenser you order specifies it is parallel flow. You can also identify the different condensers visually. The Serpentine flow condenser has around 16 rows of tubing. The Parallel flow condenser has 30+ rows of tubing. Count the rows on the condenser you already have to check if you do or don't have the parallel flow.
-Brand new 1994-1997 Receiver/Drier. You need a 94-97 receiver/drier. You can't use the 90-93 receiver/drier. The 1990-1993 receiver/drier fits both the Serpentine flow and Parallel flow condenser, but it is not fully compatible with the R134a refrigerant. The refrigerant can chemically react with the R12 desiccant inside the 90-93 drier and cause corrosion to the system and contamination to the R134a refrigerant. Also, the internal volume between the R12 Drier and R134a Drier is different. The receiver drier acts as a reservoir to store excess refrigerant when ambient temperature differs. Maintaining the proper reserve volume is dependent on refrigerant charge and refrigerant type.
-Couple ounces of ND-9 refrigerant compressor oil or equivalent
-Proper radiator ducting, fan cowling, and functional cooling fans for efficiency.
-New O-rings compatible with R134a and ND-9 oil. These are typically green. Ideally, every single o-ring in the system is replaced.
The bare minimum for a functional AC conversion is the proper Parallel Flow condenser, the R134a refrigerant, a new receiver/drier, and service port fitting adapters.
When all these parts are installed in your car, you can service the system with R134a. If you plan to DIY the servicing, you'll need a set of R134a manifold gauges, and a compatible vacuum pump. Pretty cheap equipment from Harbor Freight.
Don't vent refrigerant to the atmosphere. This write-up is done all under the assumption your system is already empty and/or properly recovered in recovery tanks.
To service the system, it must first be free of any positive pressure. The manifold gauges should be connected to the adapted high side and low side service ports. A constant vacuum must then be pulled on the system for at least 30 minutes. I recommend vacuum pumping for 60 minutes to be thorough. This will ensure any vaporized water, or moisture, or other non-condensibles have been removed from the system. Any contamination in the lines will cause operating problems. After a constant vacuum has been pulled, the pump should be shut off. A leak check must be performed next. Take a picture of the Low side manifold gauge after turning off the vacuum pump. Return to check the gauge in 30 minutes. If the needle has not moved in comparison to the photo you took, then the refrigerant system has no leaks, and you can now add oil and refrigerant with your manifold gauges through the Low side port only.
ChrisFix honestly has a great video of how to service and charge R134a refrigerant. Better tutorial than I can explain here through text.
Equivalent R134a capacity by mass will be about 75% less than what an R12 system takes. Expect to charge this converted system with about 18oz - 22oz. The easiest way to get the correct refrigerant level is to simply add enough R134a to the system with the compressor running (engine at idle, AC on max, windows down) until vent temperatures fall under 45 degrees farenheit, and low side manifold pressures at idle fall between 20psi-40psi. High side pressures should be around 200-220 psi. If you have a sight glass, you shouldn't see any bubbles or foam behind the glass. If you see bubbles, the refrigerant level is low. Keep adding until bubbles in the sight glass are gone and it looks crystal clear through the glass.