Abstract: In its first use in simulating engine operating conditions, the Thermo-oxidation Engine Oil Simulation Test (TEOST®) was applied to the study and deposition measurements of engine oils under turbocharger operating conditions in a technique called Protocol 33C.  Building on a previous paper, the authors present data and discussion on the factors involved in the high temperature cyclic test conditions of the TEOST and reemphasize differences between it and steady-state oxidation tests. Some new studies are discussed which involve application of the TEOST apparatus at lower temperatures using simulated crankcase gases. These are believed more reflective of piston deposit conditions. Preliminary studies indicate that results from this new lower-temperature protocol correspond to known reference engine oils in regard to piston deposits and other measured factors. TEOST