TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: In the method (M1), the step of separating involves mixing the engine oil sample with a polar solvent to form a mixture, agitating the mixture, and permitting the mixture to settle for at least 24 hours, where the polar solvent has polarity index of greater than or equal to 5, and the polar solvent comprises 1-4C alkanol (preferably methanol). The step of analyzing involves injecting the polar component into GC/MS, where GC column of the GC/MS comprises phenyl (5%) and dimethylpolysiloxane (95%); and heating the sample during the GC process to a first predetermined temperature of 200 degrees C for 45 minutes. The step of analyzing further involves heating the sample to a second predetermined temperature after the first predetermined temperature, where the second predetermined temperature is 310 degrees C for greater than or equal to 20 minutes. In the method (M2), the sample of the engine oil is a first sample of first engine oil and the detecting provides a first amount of the biodiesel chemical components (A2), where the method further involves detecting a second amount of the biodiesel chemical components related to the presence of biodiesel chemical components in a second sample of second engine oil; separating the second sample into a polar component and non-polar component via extraction with a polar solvent having polarity index of greater than or equal to 5; analyzing at least one of the polar or non-polar component for the second sample for the biodiesel chemical components, where the second sample is distinct from the first sample; and comparing the first amount of the biodiesel chemical components to the second amount of the one or biodiesel chemical components. The comparing further involves determining a difference between the first amount and the second amount of the biodiesel chemical components. The analyzing comprises Gas Chromatography and Mass Spectrometry (GC/MS) and optionally flame ionization detection (FID). In the method (M3), the analyzing involves injecting the sample into Gas Chromatography/Mass Spectrometry (GC/MS), where Gas Chromatography column of the GC/MS comprises phenyl (5%) and dimethylpolysiloxane (95%). The analyzing includes heating the sample during the GC process to a first predetermined temperature is 100 degrees C for t 1 minute. The analyzing further involves heating the sample during the GC process to a second predetermined temperature after the first predetermined temperature, where the second predetermined temperature is 200 degrees C for 1 minute; heating the sample during the GC to a third predetermined temperature after the second predetermined temperature, where the third predetermined temperature is 310 degrees C for 40 minutes; flame ionization detection (FID) of the fuel sample. Preferred Components: The biodiesel chemical components (A1) are selected from plant sterols, fatty acid methyl esters, and/or cetane. The diesel chemical component is biodiesel chemical component (A1) selected from stigmasterol, campesterol, methyl heptadecanoate, arachidic acid methyl ester, behenic acid methyl ester, gadoleic acid methyl ester, lignoceric acid methyl ester, erucic acid methyl ester, soybean plant phytosterol, and/or rapeseed plant phytosterol. The biodiesel chemical components (A2) is selected from coolant, cetane, methyl heptadecanoate, arachidic acid methyl ester, behenic acid methyl ester, gadoleic acid methyl ester, lignoceric acid methyl ester, erucic acid methyl ester, soybean plant phytosterol, rapeseed plant phytosterol, stigmasterol, and/or campesterol. EXAMPLE - No suitable example given.