The use of multigrade (V.I. improved) oils in automotive engines has increased significantly in recent years. However, the performance of these oils in terms of factors such as oil economy, wear, and noise, is not always equal to that of single grade oils.
Although the initial viscosity of multigrade oils is related to both the base oil and the V.I. improver, the viscosity decreases with use, with the primary factors determining the magnitude of the change being the degree of shear and the characteristics and concentration of the V.I. improver used. This decrease in viscosity has been assumed to be the cause of the decreases in oil economy that may occur with oil use. However, viscosity changes are not believed to be the primary factor responsible since similar oil economy changes have also been observed for single grade oils. Nevertheless, the characteristics and concentration of the V.I. improver used can be a significant factor influencing oil economy.
Engine wear and noise generally decrease with increasing base oil viscosity. However, the addition of V.I. improvers may not reduce engine wear as much as would be expected from the resulting increase in oil viscosity. Also, preliminary data suggest that the viscosity contributed by a V.I. improver is less effective than base-oil-contributed viscosity in reducing engine noise.
Only selected examples illustrating multigrade and single grade oil economy, wear, and noise comparisons are included in this paper. Additional research is clearly needed to provide a better understanding of V.I. improver characteristics and their relationship to engine oil performance.