The spigot in the bath tub has a nylon tube that fits over the copper delivery pipe. You screw the spigot, which has a water blocking device for the shower, on to this. That nylon tube has two o-rings which hold the back pressure when the water is blocked.
Hardware store said use plumbers grease on these rings. Won't grease cause the "rubber" to deteriorate? I put silicon grease on them instead. Seems to work fine .. now. Was that ok to do? I assume it was. Is plumbers grease really recommended for o-ring lube?