Putting better bulbs into the stock housings is mostly considered an improvement over the stock lighting. It has been debated on the CZ28.C message board many times what the best bulbs to use are. Since this is my website, I am going to give you my personal opinion, that reflects that of many others as well. For the stock housings, I would say that the Sylvania or Osram Silverstars are about as good as any other bulb you can use. These bulbs are very white in color, and also very bright. The Sylvania's burn at 4000k while the Osram's burn at 4300k. The Sylvania bulbs are most commonly available at Advance Auto Parts stores.
You will also here it debated that the higher wattage bulbs are the best (~100w). They are also very powerful and you should consider them and research that option a little more before rushing out and getting the Silverstars. I am going to try and get a collaboration of bulb information off of the CZ28.com message board put up here, but until then - use that little red search button at the top of every page on the board.
For more notes on this at this time, navigate away from this site and go the guide below. It is not the site of choice - but the other one moved, so this will have to do for now.
