The New York Central R.R. Technical Research Dept., finding the procedures currently employed by industry in evaluating the quality of diesel lubricating oils to be inadequate, has designed a test procedure which places particular emphasis on the oxidation stability of an oil. Tests previously employed by the railroad's laboratory were inadequate for separating the various oils into good and bad categories. Most oils submitted for laboratory approval passed, but later many caused serious engine distress in the field. In addition to this, the tests used were extremely time-consuming, sometimes running as long as several weeks. The laboratory's researchers, after months of work, were convinced that the resistance of an oil to oxidation was highly indicative of its general performance in a diesel engine; field correlation substantiated this theory to their complete satisfaction. The procedure was accelerated and a simple mathematical interpretation of results followed the construction of a unique apparatus in which this laboratory work was performed. A package test was evolved which reduced running time to about one-tenth of that needed for the most rapid of those used previously. The quality of diesel lubricating oils purchased by the railroad improved and several suppliers felt the test to be so indicative that they use it in their own laboratories, while one large refiner currently is using it in the evaluation of used lubricating oils. This paper describes the test procedure and the design of the apparatus used, and supplies data required to assign numerical demerit ratings to oils tested in this new apparatus.