The invention provides an additive concentrate which comprises an oil soluble hydrocarbyl poly(oxyalkylene) aminocarbamate having a number average molecular weight (M.sub.n) in the range 600 to 10,000 having at least one basic nitrogen atom wherein said hydrocarbyl group contains 1-30 carbon atoms, and an oil soluble hydrocarbyl amine of formula R--NH.sub.2 wherein R represents a group R' or a group R'--CH.sub.2 --, wherein R' represents a hydrocarbyl group having a number average molecular weight (M.sub.n)in the range 750 to 6,000; a gasoline composition containing such additive concentrate; and its preparation; and a method of operating a spark-ignition engine using such gasoline composition.
