We have determined the optimal conditions for quantitative analysis of dithiophosphate (DTP) additives to oils by thin layer chromatography: the use of chromatographic plates with finely porous sorbents, preliminary cleaning with hexane, elution with hexane-acetone mixtures, detection by an aqueous PdCl2 solution. We have studied the nature of the dependence of the area of the chromatographic spot on the relative additive content and the composition of the eluent (the hexane: acetone volume ratio). We demonstrate the feasibility of quantitative evaluation of the degree of depletion of DTP additives in motor oils by thin layer chromatography. We studied the dynamics of the decrease in DTP additives in two brands of motor oils during use.