NOVELTY - The method involves ageing (S4) contents (MIX) of a reactor (1) during given ageing duration (FD) and at given ageing temperature (TV). The ageing of the contents of the reactor is controlled (S5) by measuring characteristics of ageing of contents of an engine. Engine oil (HL) added with fuel (BC) is correctly packaged when final quantity of additive is higher than quantity additive of a threshold (Sad) in the engine oil, and final quantity of the fuel is higher than of the threshold fuel quantity (Sbc) in the engine oil.    USE - Method for packaging engine oil added with fuel e.g. petroleum with biofuel, lubrication with an engine i.e. petrol engine, of a vehicle (all claimed).    ADVANTAGE - The method enables conditioning the engine oil in presence of petrol with the fuel, thus allowing an artificial ageing of the oil used for lubrication of a petrol engine, allowing reproduction of usage conditions of the engine to maintain a mixture of oil added with the fuel remaining stable in time, and correctly evaluating performances of oil/gas mixture after ageing duration. The method enables maintaining properties of the lubrication between two draining valves due to selection of oil, thus increasing durability of motorizations while reducing increased costs so as to promote customer quality, and hence preventing risks related to quality of the biocarburants.    DETAILED DESCRIPTION - The additive of acceleration of the ageing of the engine oil is the additive selected from a group consisting of metallic oxides, hydroperoxides, agents of nitro-oxidation and iron acethylacetonate.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart illustrating a method for packaging engine oil added with fuel.    Fuel (BC)    Ageing duration (FD)    Engine oil (HL)    Contents (MIX)    Thresholds (Sad, Sbc)    Step for ageing contents of reactor during given ageing duration and at given ageing temperature (S4)    Step for controlling ageing of contents of reactor (S5)    Ageing temperature (TV)    Reactor (1)