Automatic transmission fluids (ATF) should be oxidatively stable so that their frictional properties are maintained as the fluids are aged. To test the oxidative stability of ATFs, automobile manufacturers have created oxidation tests in which ATFs are aged in operating transmissions. In these tests, the total acid number (TAN) of the oil is measured throughout the test, and at the end of the test the TAN of the oil must be below specified limits. In general, oxidation of oils occurs by formation of free radicals that can react with the oils to form acidic species that are detected by the TAN of the used oils [1, 2]. Peroxides also form when an oil is oxidized and the peroxides can react with the oil to form acids [1,2]. Base oil structure, presence of wear metals, and the amount of oxygen dissolved in the oil can all affect the oxidative stability of oils [1,2]. Therefore, we investigated how each of these three factors affect changes in TAN as oils are aged in the GM cycling and GM oxidation tests (GMOT). Base oil structure is the major factor affecting the oxidative stability of ATFs. In particular, we have found that the cyclo-paraffin concentration in the base oils used to formulate ATFs can be related to oxidative stability. The lower the number of cycloparaffins in the base oil, the better the oxidative stability of the ATF.

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