[Amps] Oil-caps delating 2/3 ratings?

[Han Higasa]

Recently an old man told me that all of the caps are energy storage jobs
and they are usually derated for continuous duty to about 2/3 rating.
I thank him very much.

I am using cheap 32uF 4500V GE non-pcb caps.
Should I use it at 3000V maximum? 
Fortunately I am using this potential level now.
If someone use 16uF 7500V Arioso caps like Henry,
maximum working voltage should be 5000V?

Would you please tell me the reason for the "2/3 derating" 
practically when using them for our linear amps?

For a long time I had a question why GE had not indicate working 
voltage and test voltage but only "4500V".

Additionally anyone did a destructive/non-destructive test on it?
Seemingly they are sustainable at 5kV, but if the number 2/3 work
their life should be very short.

de Han JE1BMJ