The paper presents selected information from a research project focused on the diagnosis of engines using a worn lubricant. It introduces sorted information, description and methodology of the project solution and results of practical experiments including the determination of criteria for tribodiagnostic evaluation of the technical condition, the mathematical model for evaluation and prognosis of the lubricant degradation process based on the balance between the formation and loss of wear products, for the evaluation of technical condition (serviceable, limit and emergency conditions), and the design of the optimal system of implementation of tribodiagnostics of selected engines.