Chevron is not sure whether the transition to S15 (ULSD) in 2006 will affect fuel system elastomers (e.g., O-rings). Any change will depend on the elastomer material and age, as well as the change in seal swell characteristics of the particular fuel being used, relative to the fuel the vehicle has been using. Experience in transitioning to S500 (500 ppm) in 1993 suggests end-users should be proactive. Specifically, maintenance records should be reviewed to ensure that fuel system elastomer seals have been changed out at recommended intervals with recommended materials.