I am trying to restore convertible top cylinders. There is an O-ring at the top and both are leaking. I extricated them but it is impossible to tell what material they are made of. I figured something like VITON or BUNA-N would be applicable. It must seal against ATF and fairly high pressure. Someone suggested the green HNBR stuff they use on AC systems. I have used those for different things but I am not sure they are ATF resistant.