At high temperatures, oil reacts with R-12 to produce a darkened oil and R-22. Stable oil is one that undergoes this reaction slowly and vice versa. In testing the stability of oils, some laboratories rely primarily on color and others on R-22 formation. Both methods have proven to be satisfactory.

Until quite recently, practically all domestic refrigeration oils were of mineral origin. Synthetic oils are now finding increasing usage and in preliminary tests with one of these, it was noted that the relationship between R-22 formation and oil discoloration was quite different than for mineral oils. At comparable R-22 levels, there was much less discoloration of the synthetic oil. This seems to indicate that the R-22 formation criterion for mineral oil stability does not apply to synthetics. To clarify this matter, work was undertaken to put on a quantitative basis the relationship of R-22 formation to color for both synthetic and mineral oils.