A method of testing a synthetic lubricating oil from a compressor cooling system for residual content of mineral oil after the refrigerant Freon 12 has been replaced by coolant 134 A is performed by pouring a measured quantity of the lubricating oil into the same measured quantity of pure methanol which is shaken and observed after some time. If appropriate, a small quantity of dye can be added. After some time, a dividing surface is formed between the oil and the oil dissolved in the methanol if the residual content of mineral oil in the synthetic oil is more than roughly 1%.
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