Abstract: The operational fluid according to the invention is well suited for the lifelong lubrication and cooling of combustion engines. It comprises as the basic fluid a polyalkylene glycol produced from ethylene oxide and propylene oxide and an additive mixture comprising a) 0.012-1.0 percent by weight of a reaction product of diphenyl amine and 2,4,4-trimethyl pentene; b) 0.01-1.0 percent by weight of a pentaerythritol partially or completely esterified with an alkyl-substituted p-hydroxy propionic acid; c) 0.01-1.0 percent by weight of a mono- or di-(C4-C8) alkyl phosphoric acid (C10-C15) alkyl amide; d) 0.01-1.0 percent by weight of triphenyl thiophosphate; e) 0.01-0.1 percent by weight of a tolutriazol amino-methylated with a straight-chain or branched alkyl group with 2-10 carbon atoms, and/or f) 0.01-0.1 percent by weight of 1H-benzotriazol.