Several factors to consider. The designed current for an average H4 is 12.8V. Much more than that will significantly reduce life. If you've upgraded your healight harness, you're difenitley putting more than 12.8V to the bulb. Check the current at the bulb and see where you're at. Also, the higher the wattage (on average) the shorter the life. Heat is also a killer. If you're housings are not vented properly, heat will build up and the bulb life will suffer. Might check that you're housings are mounted right side up. I've seen where they are installed upside down......I don't see how anyone could NOT realize that they had done that.....but it's easy enought to verify.
I ran some Silverstar lamps in my XJ and loved them. However, I couldn't get anymore than about 7-8 months out of them. I run an upgraded headlamp harness and get signifcatly more than 12.8V at the bulb. So, I went with some LL (long life) lamps and they've been working ever since.
Something else to think about for those of you running high wattage bulbs. The housings that you're putting these high wattage bulbs into are likely not rated for that kind of bulb. You could....could, end up with some junk housings. Especially if you've got some cheapie housings. You run the risk of the metalizing (shiney suff on the inside) outgassing and then you'll end up with some rainbow colored housings. They may even get a smoke tint to them on the inside. The housings may also become brittle and fall apart or warp....... Not to mention fire. That'd be extreme but it is possible. I've seen just about every failure mode you can imagine in all forms of severity.
For those that don't know: I am employed by an OEM lighting manufacturer. I work in the QA lab and test this stuff on a daily basis.
