LARGE differences in cold-starting behavior have been found in engine cranking tests on 10W-30 oils formulated with different V.I. improvers. Oils of identical viscosities at 210 F and ASTM extrapolated viscosities at 0 F cranked faster or slower than conventional 10W mineral oils, depending on the particular V.I. improver-base oil combination used in their formulation.
It has been found that a substantial portion of the viscosity imparted to the oil by the V.I. improver is temporarily lost under the high shearing stresses encountered in cold engine startup. Thus, the cold-starting performance of a V.I. improved oil approaches that of the base oil from which it is blended.
It appears from this work that the SAE system for defining the limits of winter-grade oils, while adequate for mineral oils, needs improvement for polymer-thickened multigraded oils. Laboratory viscosity measurements under high shearing stresses are now being conducted in an effort to obtain a simple technique for estimating the cold-starting performance of polymer-thickened oils.