AFAIK, 95-98 used the black impellar which is very prone to failure. 99-00 used the white / tan improved impellar, less prone to failure, but still plastic. I put in the metal WP at 63K for peace of mind. I noticed no increase in temps. My temp guage has no letters on it, but it seems to be in about the same place as it was before. I did not flush the coolant. I wanted to, but I barely had time to do the WP swap. IMHO, I don't think it's all that critical to flush it unless the WP has failed and you need to get the impellar debris out. I plan on flushing in September.
