the formula: RSi(CH3)2 O(SiR2 O)p Si(CH3)2 R wherein: R is hydrogen or the methyl, ethyl and/or phenyl radical, and only one hydrogen atom is bonded to a respective silicon atom, at least 0.2 Si-bonded hydrogen atoms are present on the average per silicon atom and at least three Si-bonded hydrogen atoms are present per molecule; and p is a whole number having a sufficient value so that the viscosi