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?