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There are two distinct sources of variation when making viscosity measurements of lubricating oils at low temperature and low shear stresses -- the variations attributable to the instrument and the variations from the materials being measured. Oil variation arises primarily from the ‘crystal forming’ characteristics of the fluids tested at low shear stress. In this study, the viscosities of several fluids were measured at low temperature and low shear stress using the Mini-Rotary Viscometer (MRV), which contains nine separate viscometer cells that are cooled and controlled in unison. The testing procedure was to measure the viscosities of nine samples of each oil in a single test. Data were analyzed to compare the variances and standard deviations of the normalized viscosities. The results showed that the precision of the test on PAO-type oils was better than that for fully-formulated commercial oils.

Current specifications for automatic-transmission fluids and gear oils have viscosity limits which are determined by ASTM D 2983. However, that test is plagued by poor precision. This paper describes the development of a method using the Mini-Rotary Viscometer to make the determination of apparent viscosity at the same nominal shear stress as ASTM D 2983. In this test procedure, samples are cooled in a manner similar to that described in ASTM D 2983. Experimental data were obtained on a mixture of 17 automatic-transmission and gear-oil fluids that included a number of different formulation strategies and commercial products. The results of this method yield a nearly one to one correlation with the results determined by ASTM D 2983.