American Petroleum Institute. The primary oil and natural gas trade association in the United States. API operates a voluntary licensing and certification program that allows engine oil marketers to use the API Engine Oil Quality Marks if their products meet specific requirements. CI-4 oils that also carry the classification CI-4 PLUS are formulated to provide a higher level of protection against soot-related A measure of a fluid's resistance to flow. A fluid with a higher viscosity flows less easily. increase and viscosity loss due to shear in vehicles powered by diesel engines. As of September 1, 2004, oils that meet the requirements for API Service Category CI-4 PLUS and that are properly licensed may display “CI-4 PLUS” in the lower portion of the API Service Symbol in conjunction with API Service Category CI-4 in the upper portion.
Oils that satisfy API Service Category CI-4 PLUS are superior in performance to those meeting API CF-4, CG-4, CH-4 and CI-4, and can effectively lubricate engines calling for those Service Categories.