Thin Layer Activation (TLA), a radionuclide technique, was recently applied at the Research Center of ELF Solaize, France, to perform on-line monitoring of wear on diesel car engines. Measurements aimed at better understanding how wear develops on cam lobes as a function of various parameters such as lubricant formulation, soot concentration, and oil aging. The engine itself (passenger car diesel engine) was operated under severe steady state conditions (speed, torque, oil temperature and smoke) and on-line measurements were performed by applying two independent methods: direct measurement of material loss from cam lobes (inlet and exhaust valves) and concentration measurement of worn debris in oil or in the oil filter. These methods have proved to be very repeatable and absolute wear values given by TLA were consistent with those measured by profilometry.Experiments using end of test sooted oils have shown a very poor correlation between observed TLA wear rate and camshaft average wear results of the previous 50 h engine tests. Experiments on fresh oils have shown the existence of a threshold effect of soot on wear associated with a depletion in additive efficiency; this threshold depends on oil formulation and highlights the importance of the draining process as the best way to efficiently protect the engine.The impact of oil formulation has also been demonstrated. For extending the oil drain interval, we have shown that it is crucial to choose an oil that gives the lowest wear rate before and after draining. Other effects such as the filter collection efficiency or the impact of cam configuration (inlet or exhaust valve) have also been investigated, confirming the complexity of cam wear mechanisms.