Hi Jules,
I've used the Philips X-Treme H4 bulbs, which are the same wattage as the standard bulbs. I've found that they offer a genuine improvement in the penetration (Oh er Matron) and a much clearer pure white light.
The down side is that I average about 500 hours of use per bulb, which consider I ride at least two hours a day then they don't last anywhere near as long as the oriinals.
However as the beam is so much better then I'll continue to use them.
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