As the use of carbon-neutral fuel (CN fuel) becomes more widespread, the issue of contamination of lubricating oil by CN fuel itself and by water produced during the combustion process of CN fuel is becoming a problem. The contamination and deterioration caused by the mixing of CN fuel and water with engine oil can lead to increased wear and seizure. Therefore, it is necessary to clarify the friction and wear characteristics when CN fuel and water are mixed, and to explore the formulation and combined effects of additives that can demonstrate sufficient lubricating performance even in the presence of these substances. The aim of this research is to clarify the formation process of tribo-chemical reaction films (tribofilms) and their friction and wear characteristics in the presence of CN fuel and water.