rtv (room temperature vulcanizing) sealants for use on automobile engines must be "oxygen sensor safe". yes it is true no matter how much sealant you dump between two mating surfaces, it will still leak if the two mating surfaces are not clean and dry. i.e., the surfaces must be free of any trace or film of oil. even a fingerprint of an unclean hand will cause a seepage of fluids after the sealant has cured. the surfaces can be cleaned with lacquer thinner or acetone or denatured alcohol to assure a leak proof gap.