Method for optimizing the interval of draining the engine oil of a diesel-type internal combustion engine comprising at least one cylinder pressure sensor and a control unit, characterized in that a percentage by weight of polluting particles (TMC(Tn)) in the engine oil is estimated for an operating period of the engine (Tn), as a function of the value of an angle, in crank angle degrees, for which 90% of the fuel has been consumed as a function of the pressure measured in the cylinder, of a value of the opaqueness of the exhaust fumes and of the quantity (Qe) of fuel injected into the combustion chamber; wherein, to estimate said percentage by weight of polluting particles (TMC(Tn)): - there is estimated, for each speed/torque operating point (i) of the engine, a level of polluting materials present in the engine oil after a relatively long predetermined period that the engine would spend at said operating point (i), - there is deduced therefrom an estimation of a level of polluting materials (dTMC) added to the oil of the engine during a relatively short predetermined elementary period (dT), during which the engine operates at a substantially stabilized speed/torque operating point (i), - there are added all of the values (dTMC) of the level of polluting materials added to the engine oil during each elementary period (dT) during which the engine operates at a given stabilized operating point (i), between an initial time at which the level of polluting materials present in the oil is zero and a current time corresponding to the operating period (Tn) of the engine, in order to obtain the final value (TMC(Tn)) of the level of polluting materials in the engine oil for said operating period (Tn), and - said final value (TMC(Tn)) of the level of polluting materials in the engine oil is compared with a threshold value, when the final value (TMC(Tn)) of the level of polluting materials in the engine oil is greater than said threshold value, a drainage instruction is emitted and, when the final value (TMC(Tn)) of the level of polluting materials in the engine oil for the operating time period (Tn) is less than said threshold value, the final value (TMC(Tn)) of the level of polluting materials is then incremented by the value of the level of polluting materials added to the oil (dTMC) during the following elementary operating period (dT) of the engine. - there is estimated, for each speed/torque operating point (i) of the engine, a level of polluting materials present in the engine oil after a relatively long predetermined period that the engine would spend at said operating point (i), - there is deduced therefrom an estimation of a level of polluting materials (dTMC) added to the oil of the engine during a relatively short predetermined elementary period (dT), during which the engine operates at a substantially stabilized speed/torque operating point (i), - there are added all of the values (dTMC) of the level of polluting materials added to the engine oil during each elementary period (dT) during which the engine operates at a given stabilized operating point (i), between an initial time at which the level of polluting materials present in the oil is zero and a current time corresponding to the operating period (Tn) of the engine, in order to obtain the final value (TMC(Tn)) of the level of polluting materials in the engine oil for said operating period (Tn), and - said final value (TMC(Tn)) of the level of polluting materials in the engine oil is compared with a threshold value, when the final value (TMC(Tn)) of the level of polluting materials in the engine oil is greater than said threshold value, a drainage instruction is emitted and, when the final value (TMC(Tn)) of the level of polluting materials in the engine oil for the operating time period (Tn) is less than said threshold value, the final value (TMC(Tn)) of the level of polluting materials is then incremented by the value of the level of polluting materials added to the oil (dTMC) during the following elementary operating period (dT) of the engine.