[SFfiredog]

Sodium filled exhaust valves are used in nearly every air-cooled engine, especially air-cooled aircraft engines.
The main design use is to dissapate heat, not really for light weight.
That's why they are so important in air-cooled engines.
The problem with sodium filled valves is the sodium creates corrosion inside the valve stem.
Sodium (salt) corrodes metal. The older the valve, the more the corrosion, new or used.
It was common practice to replace the exhaust valves after every race on the Ford NASCAR teams 427s and Boss 429s.
Forty + year old OEM Ford valves, wether they are used or NOS, are ticking time bombs in your engine.
We used to X-ray the sodium filled valve stems to be sure they were ok to use. I found so many that were paper thin that I decided never to trust old valves, NOS or not.
I stick with stainless steel on the street and titanium on the track.
HTH,
--Mike