Cal. Business and Professions Code - BPC § 13500. Section 13500.
It is unlawful for any person to transport in any tank vehicle, for the purpose of sale or for delivery to any place where motor vehicle fuels or lubricants are stored for sale, any product referred to in this chapter unless there is firmly affixed at each outlet or valve of the tank vehicle, a metal tag, plate, or label. The tag, plate, or label shall display, in letters not less than one-half inch in height, the name and grade of the product in the tank compartment of the tank vehicle. In the case of motor oil, the SAE International viscosity number shall also be displayed on the tag, plate, or label.