The literature provides considerable evidence confirming interaction of oil and friction material on performance characteristics obtained in a wet friction unit. Both elements can be modified to meet a wide range of requirements. Furthermore, it is well established that the performance obtained with any specific oil-material combination can be modified with usage. In this report, additional evidence of interaction is offered from test of five friction materials and five oil types. How the interactions are affected by conditions of use is also explored. Data on variable energy loading, power loading, oil sump temperature, oil flow, and sump volume factors are included, not only as a design guide on methods of eliminating or minimizing performance fluctuations resulting from such interactions, but also to demonstrate the kind of variable that can produce the chemical and/or physical change of sufficient magnitude to modify results. Also demonstrated is the fact that the effects of such conditions of use are not necessarily consistent and that trends produced by a variable may be different for each oil-friction material system. It is implied that further study of such variables is required to permit scientific selection of the optimum combination for any specific design.