This paper discusses slow-speed marine diesel engine cylinder oil and the analysis of cylinder oil. The primary concern of the paper is cylinder friction which causes upwards of 20% of all internal diesel engine friction losses. A basic hypothesis is proposed, that the knowledge of basic tribology and understanding of cylinder oil significance are importan in understanding cylinder lubrication. That consists of basic lubrication theory, tribological terminology, the processes of wear and lubrication with special attention given to types of lubrication. Then, by means of drain cylinder oil chemical analysis, we can make conclusions about wear, lubrication and stress of cylinder components and that data can be used to adjust the cylinde oil settings. Further on, cylinder oil classification was examined along cylinder oil properties relevant for the effectivness of a chosen oil. After that, cylinder oil functions are analyzed. Based on that conclusions were made about the types of additives the cylinder oil needs to have in order to fulfill its function. Special attention was given to the cylinder oil feed rade and cylinder oil system. The paper used the Alpha B&W lubricator system as the example. System components were analyzed, as was the way the system achieves monitoring and control over cylinder lubrication and how advanced aspects of the system enable easier handling of cylinder lubrication. The paper focal point is liner, piston and piston ring tribology analysis. Materials, properties, and the interaction of different parts is analyzed, as is the role of the cylinder oil in sealing, cleaning, heat transfer and lubrication. Minimum oil film thicknes as a function of cylinder liner position is demonstrated for different loads. Based on that conclusions are drawn about the type of lubrication.The acquired understanding of cylinder tribology is applied to analize the chemical composition of drain cylinder oil. Observations are made about oil and component condition based on drain oil chemical composition and the mechanism of optimizing cylinder oil feed rate is demonstrated. In conclusion, the acquired understanding is put in to the contex of modern international martime laws and markets and conclusions are drawn about the future develpment of ships systems and cylinder oil.