The present collection of papers on the subject of the Relationship Between Engine Oil Viscosity and Engine Performance is Part IV of a series of symposia on this subject which have been sponsored by the Society of Automotive Engineers, Inc. and the American Society for Testing and Materials. Several different facets of the complex relationship between this major source of the world's mechanical power—the engine, and the engine's sole protection against immediate and catastrophic breakdown—the engine oil, have been studied, presented, and discussed in the previous symposia. In this, the Fourth Symposium, the authors: 1) look at the effect of viscosity on oil supply to the engine at low temperatures and low shear (W. Wiemann and W. J. Bartz; 2) present high-shear studies of the viscous properties of engine oils in the laboratory (F. R. Banks, S. Kulik, and W. C. Pike; N. van Os, A. Bos, D. van Namen, and J. de Rooij); 3) apply our present level of technical understanding to results from engine studies (J. du Parquet and A. Godet; M. L. McMillan, R. C. Rosenberg and C. K. Murphy); and 4) question the role of viscosity in engine fuel economy (P. J. Ghirla and R. K. Smith). Finally, in a review of a major colloquium on the subject of the viscometry of multigrade oils held late in 1977, W. J. Bartz summarizes its technical impact.