Short Life of Oil CAP only 250 Hours??

I assume your talking about the Jensen oil cap web site. The specification listed is the breaking point, when subjected to maximum temperature for 250 hours.

A few rows up from where you got your numbers, the maximum temperature is listed as 185 degrees F.

Unless the cap is put in an extremely high bias circuit or next to a tube, it is unlikely to be subjected to this temperature extreme. Many are purchased for speakers where this is not an issue.

the oil caps in my speakers' crossovers have lasted for 25 years + counting

Such a spec is only for the maximum temperature the part is rated for, kind of a 'failure' rating - not the life of the part for the recommended temperature range at which it is intended to be used. That temp will be significantly lower, and the resulting part life much longer. See if you can do a little more research - from what I've seen, there is often another projected lifetime rating given for a 'normal' average temperature.

Aside the topic, does anyone know the sonic benefit of silver foil over copper foil? I am using copper foil, but wonder if it's worth $$ to replace it with silverfoil.

Regarding my question, Han at Jensen was kind enough to response my email so I will post it here just FYI.

"The lifetime test has nothing to do with the actual life time of the capacitor.
In many standards for electrical components you find type tests, which are accelerated life tests to show if the component has a certain expected life time. Then you measure electrical parameters before and after the test, and then the values must only change within a certain range.

Such a lifetime test indicates whether the component is OK or not.

The expected lifetime for our paper capacitors is minimum 5 years for aluminium foil capacitors, and 10 - 15 years is very common. For copper foil and silver foil it can be shorter (5 - 8 years) due to the properties of the
metals.

Best regards
Hans J. Jensen