Base Oil and Additive Effects in the Thermo-oxidation Engine Oil Simulation Test (TEOST)

The new ILSAC GF-2 specification for passenger car motor oils includes the TEOST bench test as an indicator of the deposit forming tendencies of motor oils. Base oils, base oil / additive component mixtures, and fully formulated motor oils were run in the TEOST apparatus. Some of the used oils and deposits were analyzed using FTIR spectroscopy and x-ray photoelectron spectroscopy. The results of these experiments show that the base oil fraction of the lubricant, which may range from conventional solvent refined mineral oils to high VI hydrocracked or synthetic PAO, contributes very little to the TEOST's total deposits (< 5 mg, < 60 mg = “pass”). In contrast, lubricant performance in industry standard gasoline engine tests and taxi fleet tests is typically highly sensitive to base oil composition. Furthermore, additives designed to reduce wear, provide anti-oxidancy and lower friction substantially increase the amount of TEOST deposits. This suggests that the deposit forming mechanisms in the TEOST, originally correlated to turbocharger deposits, may be different from those observed in tower temperature regions of gasoline engines.