Results from this study indicate that the reproducibility of the CEC Standard Test Method, CEC-L-14-A-88, a diesel injector mechanical shear stability test for engine lubricants, is not applicable to certain engine lubricants. The test is subject to a material bias which is caused by operational parameters-the allowance of a broad valve opening pressure range - and the use of a relatively insensitive calibration fluid. The magnitude of the bias depends on VI improver molecular weight and molecular weight distribution, as determined by gel permeation chromatography, otherwise known as size exclusion chromatography. The bias can be minimized or eliminated, however, by determining viscosity loss at a specified gauge pressure. It was thereby possible to improve reproducibility and obtain excellent correlations between diesel injector and both gasoline and diesel laboratory engine tests results.