A lead acid battery separator comprising: a porous membrane having a residual oil content of greater than or equal to 10%, which comprises at least one polyolefin, one or more silicas having an average particle size from 15 μm to 25 μm, and an additive represented by a compound of Formula in which: R 1 = H , − CH 2 k COOM 1 / x x + , or − CH 2 k − SO 3 M 1 / x x + , preferably H, where k = 1 or 2; M is an alkali metal or alkaline-earth metal ion, H + or NH 4 + , where not all the variables M simultaneously have the meaning H + ; and in which either (a): • R is a hydrocarbon radical with 10 to 180 carbon atoms, which may be interrupted by 1 to 60 oxygen atoms; • n = 1; and • m = 0; or in which (b): • R is an alkane radical with 50 to 750 carbon atoms; • n = 0; • m is an integer from 10 to 200; and • x = 1 or 2; the ratio of oxygen atoms to carbon atoms in the compound according to Formula (I) being in the range from 1:1.5 to 1:30; wherein the lead acid battery separator has a basis weight of 130 g/m 2 or less, and wherein the lead acid battery separator has a stiffness in the cross-machine direction of 25 mN or greater measured as described in the specification; wherein the lead acid battery separator has a residual oil content of less than or equal to 20%. a porous membrane having a residual oil content of greater than or equal to 10%, which comprises at least one polyolefin, one or more silicas having an average particle size from 15 μm to 25 μm, and an additive represented by a compound of Formula in which: R 1 = H , − CH 2 k COOM 1 / x x + , or − CH 2 k − SO 3 M 1 / x x + , preferably H, where k = 1 or 2; M is an alkali metal or alkaline-earth metal ion, H + or NH 4 + , where not all the variables M simultaneously have the meaning H + ; and in which either (a): • R is a hydrocarbon radical with 10 to 180 carbon atoms, which may be interrupted by 1 to 60 oxygen atoms; • n = 1; and • m = 0; or in which (b): • R is an alkane radical with 50 to 750 carbon atoms; • n = 0; • m is an integer from 10 to 200; and • x = 1 or 2; the ratio of oxygen atoms to carbon atoms in the compound according to Formula (I) being in the range from 1:1.5 to 1:30; wherein the lead acid battery separator has a basis weight of 130 g/m 2 or less, and wherein the lead acid battery separator has a stiffness in the cross-machine direction of 25 mN or greater measured as described in the specification; wherein the lead acid battery separator has a residual oil content of less than or equal to 20%. R 1 = H , − CH 2 k COOM 1 / x x + , or − CH 2 k − SO 3 M 1 / x x + , preferably H, where k = 1 or 2; M is an alkali metal or alkaline-earth metal ion, H + or NH 4 + , where not all the variables M simultaneously have the meaning H + ; and in which either (a): • R is a hydrocarbon radical with 10 to 180 carbon atoms, which may be interrupted by 1 to 60 oxygen atoms; • n = 1; and • m = 0; • R is a hydrocarbon radical with 10 to 180 carbon atoms, which may be interrupted by 1 to 60 oxygen atoms; • n = 1; and • m = 0; or in which (b): • R is an alkane radical with 50 to 750 carbon atoms; • n = 0; • m is an integer from 10 to 200; and • x = 1 or 2; • R is an alkane radical with 50 to 750 carbon atoms; • n = 0; • m is an integer from 10 to 200; and • x = 1 or 2; the ratio of oxygen atoms to carbon atoms in the compound according to Formula (I) being in the range from 1:1.5 to 1:30; wherein the lead acid battery separator has a basis weight of 130 g/m 2 or less, and wherein the lead acid battery separator has a stiffness in the cross-machine direction of 25 mN or greater measured as described in the specification; wherein the lead acid battery separator has a residual oil content of less than or equal to 20%. <EOS>