The OEM spec for most cars including ours is about 4300K, offering the brightest, purest light. Going colder will seem brighter but really doesn't have the same illumination reach.

 

As JaHummer says, 4300K is what is spec for our cars. However I ran 5000K in my XJ for years simply because I like the color better that 4300K. As my lights seem to get dimmer every time I drive the car I am ready to replace these lamps with 5000K units.

 

Are the headlight bulbs in your car that are getting dimmer the originals?

 

Yes. I just replaced my original 17 year old bulbs on my 2001 with OEM Lexus dealer bulbs. I didn't want to go with the internet and got a deep discount, but still pricey. The OEM bulbs are the closest to white. 4300K. Once going with 5000K and higher the light is actually less bright and the color is closer to bluish. It does get subjective to the owner's taste, but my concerns were any affect on the ballists when wandering from OEM specs.

 

I'm pretty sure they are, which makes them eleven years old and well past their expected life cycle.