I have had experiences both where the Krown effected the rubber and when it didn't. It seems to come down to two things: the makeup of the rubber, (often differs from manufacturer) and the location of the seal. The hood seal seems to be the worst effected because it sits in the oil the most. On our cars, the only seal I was worried about was the lower door seals. There is also a silicone they are supposed to spray on the rubber before application to repel the oil, which they did when I went. That again comes down to the applicator. I have since wiped the rubber several times to keep the oil from sitting on and soaking into it.