I emailed Sylvania regarding the lifespan of their Silverstars. Here was their respose.
Thank you for your web inquiry. The SilverStars are a performance, premium
halogen that have a shorter life span than the standard. The shorter life span
is directly related to the performance nature of the lamps. The amethyst
colored coating on the glass contributes to the crisp white light emitted, but
also traps heat inside the capsule, causing the filament to burn out more
quickly, thus decreasing the life. As with most high performance products,
there is a trade-off between the higher performance and the life hours,
unfortunately. The ST product works harder (not burns hotter, however) to give
the whiter, brighter light and that impacts the life hours. The SilverStar
product is quoted as "lasting up to a year" (from our FAQ on the
SilverStar Frequently Asked Questions
What is the life expectancy of SilverStar?
The life expectancy of a SilverStar halogen light source is dependent on the
driver'* use of the lights, which vary by season, time of day and number of
miles driven. We estimate the SilverStar halogen lights to have a life span of
up to one year under average driving conditions. The life of the SilverStar is
further decreased if being used in a day-time running light application.
That being said, it sounds as though you received the average life from your
SilverStars. I am sorry I could not be of more assistance to you. Please do
not hesitate to contact me if you have any other questions.
So, if you get TWO Years or more from your Silverstars, you're doing.....well.....average.