The present invention is directed to a Zinc-free transmission oil composition for construction machinery comprising: (i) a major amount of an oil of lubricating viscosity, (ii) at least 0.25 wt. % of a dithiophosphorylated carboxylic acid compound, and (iii) a succinimide dispersant derived from a polyisobutylene group of 1200 molecular weight or less. Also provided is a method for lubricating the transmission of a construction machine comprising lubricating the transmission with a Zinc-free transmission oil composition comprising: (i) a major amount of an oil of lubricating viscosity, (ii) at least 0.25 wt. % of a dithiophosphorylated carboxylic acid compound, and a succinimide dispersant derived from a polyisobutylene group of 1200 molecular weight or less.