Zinc-dialkyldithiophosphate (ZDDP) additive in oil has been extensively reported to form a protective tribofilm between the moving components of engine for preventing wear. In this study, surface and interface characterizations were conducted on real piston ring/cylinder liner contacts, the interacting mechanism between oil-additive and solid surfaces under realistic mechanical and thermal conditions was predicted according to the identification of the chemical composition and structure of the tribofilm. The results demonstrate the interesting laminated hierarchy of tribofilm. This tribofilm is composed mainly of dense carbon precipitation at the top region and a thinner polyphosphate intermediate layer in contact with deformed iron oxide substrate on the cylinder liner. The depletion of polyphosphate compounds and competitive growth of solid carbon in term of lubricant's degradation was speculated. These significant findings help manufacturer to understand possible tribochemical processes occurring in the real tribosystem from the practical view, and in particular inspire researchers to develop the new strategy to improve capability and efficiency of oil-additive at sliding contacts.