The intent of industry and OEM factory fill oil specifications is to ensure lubricant pumping performance at low temperatures through rheological measurements using the Mini Rotary Viscometer and Scanning Brookfield tests. Often these tests provide conflicting information, yet lubricant formulations must be optimized to meet requirements of both tests. At the root of this issue is how test information is interpreted, since ultimately it is that interpretation that influences how specifications are set. In this paper, we focus on understanding the Scanning Brookfield test's gelation index which is part of ILSAC GF-2 and GF-3 specifications; our objective is to understand what is measured and its relation to meaningful low temperature lubricant performance.
We approach this objective by measuring the low temperature rheology of mineral oils and lubricants formulated from these oils. Our focus is on describing the relation of the physical processes of wax crystallization to its effects on rheological properties. Three examples are provided to illustrate what the fundamentals of this work reveal about lubricant rheology. In the first two examples we investigate the effects of oil composition and pour-point depressant concentration and illustrate some of the pitfalls associated with misinterpretation of rheological measurements, particularly in regard to gelation index as a measure of gelation. Some practical aspects are illustrated in the third example, a study of the behavior of a broad range of fully formulated lubricant oils. Our conclusion is that the gelation index is not an adequate relative measure of gelation in mineral oils.