Within the framework of the research done by the ＇＇Essais laboratoire de preselection＇＇ group, which depends on the Groupement Francais de Coordination (GFC), two evaluation tests are described of the tendency of an engine lubricant to form deposits at high temperature : A new microcoking test (Fig. 1), which is a static coking test subjecting an oil deposited on an aluminium alloy plate to high temperature gradients that are close to the thermal conditions encountered in the piston-ring zone of an engine. At the end of the test, the maximum stablity temperature of the oil is determined, as expressed by the temperature at which deposits appear (Fig. 2), and the aspect and quantity of deposits existing in the form of varnish are evaluated by determining a rating by means of the rating system described in the CEC M-02-A-78 method for evaluating the cleanness of engine (Table 5 and Figs. 5 and 10). The association of the two assessment methods results in a effective discrimination between different level oils. Results of interlaboratory tests concerning European reference oils for gasoline and diesel engines are given (Table 7 and Figs. 3 and 4). An improvement of a coking test on an inclined plate (Figs. 6 and 7), in which a given volume of oil flows as a thin film over a stainless-steel plate heated to a very high temperature. The deposits formed at the end of testing are evaluated according to their nature (carbon or varnish), their amount and also according to the principles of the CEC rating method (Figs. 8 and 9). Likewise, a system of video rating of microcoking plates by color recognition is described (Figs. 11 and 12). This system, called ＇＇Video-Cotateur＇＇ can accurately reproduce the visual rating (Fig. 13) while eliminating the subjective factors of this rating. The results obtained with this system make up the first phase in the route to the computer-assisted rating of engine parts (Fig. 14).