SEBASS

I would not mix r-134 with r-12 as per article your compressor will fail later on.
Try FR-12 which is colder then r-134 by 16F, safe for r-12 system, mixes well with mineral oil and has lower pressure then r-134 on the compressor. I was also looking at HC-12a but it being little flammable scared me a little with our cars being old, even thought i don't think anything would happen.


Replacement for R-134a
FR-12 can also replace R-134a in newer vehicles. If your system is not performing well in hot temperatures or at idle, you can recover the R-134a and then charge with FRIGC (FR-12). Vent temperatures and head pressures will be lower.
Proven Cooling Performance
An innovative and patented blend of affordable refrigerants, FRIGC FR-12 matches R-12's coolant profile for excellent performance with FR-12 A/C systems. It solves the high-side condenser pressure problems observed with R-134a, for greater reliability particularly in warm, humid climates. Because it doesn't strain the A/C system, FRIGC FR-12 protects the life of this equipment, for potential maintenance savings.
Why FRIGC (FR-12)?
FRIGC (FR-12) is rated A1/A1 by ASHRAE and Underwriter Laboratories. This means it is non-toxic and non-flammable. It is not explosive!!
FRIGC (FR-12) is accepted by the U.S. Environmental Protection Agency
FRIGC (FR-12) can be used successfully in more vehicles than other alternatives
FRIGC (FR-12) will not affect hoses, and o-rings
FRIGC (FR-12) needs no flushing or part changes
FRIGC (FR-12) will not put compressors under more pressure than they were designed to handle
FR-12 Versus R-134a
FR-12 has lower high and low-side pressures
FR-12 is designed to replace R-12
FR-12 is an enhanced R-134a blend
FR-12 is more compatible with the existing oil in R-12 systems
FR-12 produces a faster interior cool down
FR-12 is less susceptible to system shut-down under hot idle conditions
FR-12 system runs at lower temperatures and pressures, extending compressor and system life expectancy
FR-12 system has less system leakage because of lower pressure
No Change in Lubricant
In R-12 A/C systems, the refrigerant circulates the lubricant, mineral oil. While FR-12's direct compatibility eliminates the need to change lubricants, we recommend that you add two or three ounces of POE (polyester oil) on top of the existing mineral oil. This will improve oil circulation and enhance system durability.
Use your Existing R-12 Equipment
FRIGC FR-12 can be installed using your existing R-12 manifold gauges, recovery/recycle equipment, vacuum pumps, etc., through the use of simple adapter fittings. There is no need to make a large investment in new equipment to begin installing FRIGC FR-12 today.