Abstract: Hydrocarbonaceous fuels and additive compositions therefor which comprise: a) one or more fuel-soluble manganese carbonyl compounds; and b) one or more fuel-soluble alkali or alkaline earth metal-containing neutral or basic detergent salts. These compositions preferably contain, in addition to components a) and b) above, one or more of the following: c) one or more fuel-soluble ashless dispersants; d) at least one fuel-soluble demulsifying agent; e) at least one aliphatic or cycloaliphatic amine; and f) at least one metal deactivator. The compositions possess improved combustion characteristics (e.g., formation of less soot, smoke, carbonaceous products and/or noxious emissions), and form on combustion carbonaceous products of reduced acidity. The deposition of sludge on critical engine or burner parts or surfaces is reduced and the fuels have improved stability and demulsibility characteristics. And the fuel compositions can result in decreased fuel consumption in diesel engines.