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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.

To assist in the development of a bench shear stability test, computer models of shear degradation and thermal/oxidation (T/O) degradation processes of VI improvers in engine oils have been developed. The effects of the three elements of the process: VI improver, base fluid, and degradation device were separated. The models include adjustable parameters for each element. The models then allow physical degradation devices to be characterized by adjusting the parameters of the model of the device until the molecular weight distributions match the distributions produced by the device. Attempts were made to characterize the ASTM D 3945 test this way using molecular weight distributions as determined by gel permeation chromatography. This was done for two different types of VI improver. However, the shear device characteristics which produced a good match for one did not produce a good match for the other, and vice versa.

The low-temperature portion of SAE J300 Engine Oil Viscosity classification has been analyzed in terms of its scope and its use by domestic engine manufacturers. Also, its ability to protect against pumping failures has been analyzed. The analysis shows that the use is not in accord with the stated scope, and that protection against pumping failures is not adequate for a significant portion of engines. An alternative scope for J300 is proposed which is based on minimum-use temperatures for each grade. The results of this analysis will assist in formulating a future revision of SAE J300 based on the results of the ASTM Low Temperature Engine Performance program.

Engine oil viscosity loss was investigated in highway service and in ASTM D-3945 Procedure A (diesel injector) shear stability test. The six oils involved contained various viscosity index improvers and had D-3945 100°C kinematic viscosity losses ranging from low to high. The losses in service were highest for an oil with one of the lowest D-3945A viscosity losses. D-3945A also did not correlate with service using 150°C high shear viscosity losses. D-3945A shearing severity was modified over a wide range, but it still did not rank oils like highway service. Results indicate a need for a more meaningful viscosity-loss bench test.

A high shear rate capillary viscometer has been developed. It is suitable for measuring the viscosity of engine oils at shear rates of 105 to 106 s−1 and at temperatures to 150°C, conditions which are within the range experienced by oils in automobile engines. It has proven useful for evaluating the total shear stability of VI-improved oils and is also being used for interpreting laboratory engine friction testing.