Q: Disecting a cut-off valve?

I have a fuel cut-off valve (two positions, either on or off) that is
seeping ever so slightly. That led to me taking it apart and examining
its make-up, which leads to the following:

1. There is a thin copper washer or shim between the body of the valve
and the top. It has in ID of about 3/4". Am I correct that it's
purpose is to act as a gasket rather than a shim? I suspect that it's
designed to be distorted to match the two mating surfaces and that
means anytime the valve is taken apart, the copper gasket should be
replace. Correct?

2. Valve stem packing: I can visualize leaks along the valve stem
being controlled by either an 'O' ring or some form of packing. Is one
better than the other ? Sorry, I don't recall what was orginally
around the valve stem. If it was packing (vice the currently installed
'O' ring), is there a generic packing I can use or must it be specific
to that valve?

TIA - Mike