When an oil is employed, it ages as it is used. Reactions of oil molecules, e.g. with oxygen, cause oils to oxidize. The oxidation process can be significantly slowed down with special additives, the "antioxidants".
Synthetic oils are also not exempt from the aging process. Yet they generally have significantly longer service lives than mineral oils. The aging of an oil charge, especially in the industrial sector, is significantly affected by the time of use and the temperature that the oil experiences during its use.
In addition, impurities such as water, dust or wear particles have an accelerating effect on the oxidation behavior of an oil. Oil oxidation and the content of "antioxidants" still present in used oil can only be determined with the aid of elaborate laboratory analyses using infrared spectroscopy or the RULER.
"Bad odor" alone is not a sign of oil aging. To reduce wear, almost all oils also contain sulfur compounds, the decomposition products of which smell like "rotten eggs" and clearly mask the "rancid" odor that occurs during oil aging.
Unlike mineral oils, synthetic oils do not have "imperfections" in the molecular structure where oxygen can attack and thus initiate aging. Synthetic oils already have considerably better oxidation stability than a mineral oil with equal additives due to the uniformity of the molecules and the stable molecular structure with respect to oxygen influence. In addition, the oxidation tendency of synthetic oils is inhibited with modern antioxidants based on phenols, amines or salicilates until this aging protection has been used up.
Therefore, synthetic oils can often remain in use several times longer than mineral oils. Only if synthetic oils are heavily contaminated by combustion residues or mixed with more than 5% mineral oil, as is the case in engine operation, does the aging stability decrease.