The content of this article presents research on used and fresh engine oils. The aim of the experiment is to preliminarily develop a method for assessing the condition of engine oil subjected to service. A four-ball tester was used to compare the lubricating properties of the engine oil as one component of the tribosystem under laboratory conditions. The method used to determine the mashing load consisted of subjecting the kinematic node to a linearly increasing load with a build-up rate of 409 N·s-1 under operating conditions of approximately 20°C and a spindle speed of 500 rpm. The presented article is a continuation of the consideration of the lubricating properties of engine oils subjected to operation. The tests carried out made it possible to observe that fresh oils are characterised by their ability to carry higher loads in relation to oils subjected to service. This is evidenced by the obtained values of scuffing loads, which have a higher value for fresh oils (The average percentage increase in scuffing load for fresh oils was 62.23%). Comparing the friction torque characteristics with each other, it can be seen that the values of maximum friction torque are also higher for the fresh oils group. The modelling process made it possible to characterise changes in the tribological properties of the lubricating oil being used. In the future, the described model will be extended to include further input parameters (viscosity, contaminant content, fractional composition, etc.), which will allow a multi-parametric assessment of lubricating oil wear.