A metal-containing detergent, suitable for use as a lubricant additive, in the form of a concentrate in oil in which a basic metal-containing material is maintained in dispersion or solution in the oil by a gemini surfactant system comprising, or being derivable or derived from, a double bond-unsaturated carboxylic acid having 8 to 30 carbon atoms, the double bond or bonds of which being functionalized to carry polar groups across or on the double bond or bonds and the carboxylic acid group or groups thereof being functionalized to become an amide or ester group carrying at least one alkyl group having 4 to 20 carbon atoms.