A diesel fuel injector apparatus conforming to ASTM 6278 was used to examine the effects of mechanical shear on two vegetable oil-base lubricants, each blended with a different molecular weight ethyl cellulose p olymer to improve its viscosity-temp erature behaviour. Kinematic viscosity measurements conforming to ASTM D445 were used to determine the magnitude of viscosity loss after mechanical shearing. The lower molecular weight E45 sample was determined to have a permanent viscosity loss of 0.73% and a shear stability index of 1.56, whereas the higher molecular weight EC100 s ample was determined to have a permanent viscosity loss of 3.57% and a shear stability index of 6.51. Kinematic viscosity measurements of increasingly dilute solutions of prepared lubricants were used to determine the changes in the intrinsic viscosity of solution as an indicator of the decrease in polymer molecular weight after shearing.