The use of copolymers obtainable by - in a first reaction step (I) copolymerizing (A) maleic anhydride, (B) at least one α-olefin having at least 12 up to and including 30 carbon atoms, (C) optionally at least one further aliphatic or cycloaliphatic olefin that has at least 4 carbon atoms and is different than (B) and (D) optionally one or more further copolymerizable monomers that are different than monomers (A), (B) and (C), selected from the group consisting of (Da) vinyl esters, (Db) vinyl ethers, (Dc) (meth)acrylic esters of alcohols having at least 5 carbon atoms, (Dd) allyl alcohols or ethers thereof, (De) N-vinyl compounds selected from the group consisting of vinyl compounds of heterocycles, N-vinylamides or N-vinyl-lactams comprising at least one nitrogen atom, (Df) ethylenically unsaturated aromatics, (Dg) α,β-ethylenically unsaturated nitriles, (Dh) (meth)acrylamides and (Di) allylamines, followed by - in a second reaction step (II) partly or fully hydrolyzing anhydride functionalities present in the copolymer obtained from (I), for improving the thermal and/or oxidation stability of biofuels or preferably diesel fuels comprising at least one fatty acid alkyl ester. - in a first reaction step (I) copolymerizing (A) maleic anhydride, (B) at least one α-olefin having at least 12 up to and including 30 carbon atoms, (C) optionally at least one further aliphatic or cycloaliphatic olefin that has at least 4 carbon atoms and is different than (B) and (D) optionally one or more further copolymerizable monomers that are different than monomers (A), (B) and (C), selected from the group consisting of (Da) vinyl esters, (Db) vinyl ethers, (Dc) (meth)acrylic esters of alcohols having at least 5 carbon atoms, (Dd) allyl alcohols or ethers thereof, (De) N-vinyl compounds selected from the group consisting of vinyl compounds of heterocycles, N-vinylamides or N-vinyl-lactams comprising at least one nitrogen atom, (Df) ethylenically unsaturated aromatics, (Dg) α,β-ethylenically unsaturated nitriles, (Dh) (meth)acrylamides and (Di) allylamines, followed by (A) maleic anhydride, (B) at least one α-olefin having at least 12 up to and including 30 carbon atoms, (C) optionally at least one further aliphatic or cycloaliphatic olefin that has at least 4 carbon atoms and is different than (B) and (D) optionally one or more further copolymerizable monomers that are different than monomers (A), (B) and (C), selected from the group consisting of (Da) vinyl esters, (Db) vinyl ethers, (Dc) (meth)acrylic esters of alcohols having at least 5 carbon atoms, (Dd) allyl alcohols or ethers thereof, (De) N-vinyl compounds selected from the group consisting of vinyl compounds of heterocycles, N-vinylamides or N-vinyl-lactams comprising at least one nitrogen atom, (Df) ethylenically unsaturated aromatics, (Dg) α,β-ethylenically unsaturated nitriles, (Dh) (meth)acrylamides and (Di) allylamines, followed by (Da) vinyl esters, (Db) vinyl ethers, (Dc) (meth)acrylic esters of alcohols having at least 5 carbon atoms, (Dd) allyl alcohols or ethers thereof, (De) N-vinyl compounds selected from the group consisting of vinyl compounds of heterocycles, N-vinylamides or N-vinyl-lactams comprising at least one nitrogen atom, (Df) ethylenically unsaturated aromatics, (Dg) α,β-ethylenically unsaturated nitriles, (Dh) (meth)acrylamides and (Di) allylamines, - in a second reaction step (II) partly or fully hydrolyzing anhydride functionalities present in the copolymer obtained from (I), for improving the thermal and/or oxidation stability of biofuels or preferably diesel fuels comprising at least one fatty acid alkyl ester. <EOS>