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Two common internal combustion engine lubricant additives, overbased calcium sulfonate detergents and dispersant olefin copolymer (DOCP) viscosity index improvers, are shown to interact strongly, resulting in large (ca. 50%) increases in relative viscosity in hydrocarbon solutions. This viscosity increase is believed to result from bridging interactions from either physi- or chemisorption of DOCP functional groups onto the inorganic core of the sulfonate detergent colloid, with a resulting increase in effective polymer molecular weight. Viscometric properties of the detergent/DOCP interaction products, shear stability and the effect of competitive interactions with polyisobutenyl succinimide dispersant additives are described.