The present invention concerns a functional fluid comprising A) 1 to 99% by weight based on the total weight of the functional fluid of alkyl(meth)acrylate polymers obtainable by polymerizing a mixture of olefinically unsaturated monomers, which consists of a) 1-100 wt % based on the total weight of the ethylenically unsaturated monomers of one or more ethylenically unsaturated ester compounds of formula (I) where R is hydrogen or methyl, R1 means a linear or branched alkyl residue with 1-6 carbon atoms, R2 and R3 independently represent hydrogen or a group of the formula —COOR′, where R′ means hydrogen or a alkyl group with 1-6 carbon atoms, b) 0-99 wt % based on the total weight of the ethylenically unsaturated monomers of one or more ethylenically unsaturated ester compounds of formula (II) where R is hydrogen or methyl, R4 means a linear or branched alkyl residue with 7-40 carbon atoms, R5 and R6 independently are hydrogen or a group of the formula —COOR″, where R″ means hydrogen or an alkyl group with 7-40 carbon atoms, c) 0-50 wt % based on the total weight of the ethylenically unsaturated monomers comonomers, and B) 1 to 99% by weight based on the total weight of the functional fluid of an oxygen containing compound selected from to group of organophosphorus compounds, carboxylic acid esters and/or polyether polyols.