Abstract: Development and application of a bench test called the TEOST MHT® to simulate the oxidation and carbonaceous deposit-forming characteristics of engine oils in the hot zones of modern high-performance engines is the subject of this paper. Work was initiated because of concerns that ring belt and piston undercrown temperatures of 250°C and higher in modern high performance engines may cause a significant increase in carbonaceous deposits in these areas. In the absence of an engine test and reference oils for such ring-belt deposit evaluation, the TU3MH piston varnish rating test and its four associated reference oils as well as a Matrix of GF-2 and potential GF-3 engine oils were chosen to study correlation. Two groups of TU3MH engine stands were used to generate data through 1998, one group in Europe and one group in North America. The former, using special pistons, showed fair correlation between the TU3MH and TEOST MHT-4 protocol (R2 = 0.73) while the latter, using dealer-supplied pistons was somewhat poorer (R2=0.55). Analysis of the combined data plus further tests in 1999 using special pistons showed that the total data could be separated into two discrete groups both showing high correlation of varnish with carbonaceous deposits. Similar slopes of the two groups of data suggested a consistent relationship between the varnish and carbonaceous deposits but displaced in varnish severity by some third factor. Further analysis indicated that the third factor was associated with choice of special or common pistons. Importantly, it was indicated that significant carbonaceous ring-belt deposits may still form even when piston varnish levels are very low.