It is proved experimentally that structural factor is a measure of homogeneity or inhomogeneity of structural-group and fractional compositions of hydrocarbon oils, and the dynamics of its variation characterizes the initiation of secondary oxidative degradation processes. It is shown by IR-spectral analysis that carboxylic acid anhydrides are formed under conditions of high-temperature catalytic oxidation due to secondary processes. The nature of the secondary processes that occur in laboratory conditions and during use is identical.