Abstract: This paper concerns a bench test developed to simulate the effect of engine operating conditions on the oxidation and deposit-forming tendencies of engine oils.  The so-called Thermo-oxidation Engine Oil Simulation Test (TEOST) is carried out under temperatures and other environmental conditions identified as being significant in the internal combustion engine.  These parameters can be readily modified to reflect different aspects of deposit conditions and/or different forms of the mechanical design of reciprocating engines. The most important aspect of the TEOST is the separation of the oxidation process into the two aspects believed to be present in the engine, (1) the preparation of oxidation precursors in the so-called ‘Reactor’ representing the engine sump and other moderately heated areas of oil exposure, and (2) the ‘Depositor’ representing those areas of the engine where temperatures are such that the completion of the deposit-forming oxidation mechanism can be induced. Many variations of time, temperature, oil flow rate, cycling rate of oil temperature, and conditions of oxidation precursor formation were investigated. These ultimately let to a test protocol showing particular promise regarding the evaluation of modern engine oils.