A lubricity additive is a fuel treatment that restores the natural lubricating properties removed during ULSD refining. The sulfur removal process that makes diesel cleaner also strips compounds that protected fuel system components from wear. Lubricity additives create a protective film on metal surfaces in pumps, injectors, and fuel lines. This is especially critical for engines with Bosch CP4 pumps, which are vulnerable to low-lubricity fuel.