I wouldn't flush with other fluid. I used denatured alcohol, then put in the fluid to flush that out. Took about 4 bottles of fluid. Ok, here's what I learned on the different fluids, the short version:
DOT 3 & 4 - mostly used for street cars
High Temp DOT 4 - mostly for track cars
DOT 5 - a silicone based fluid that has a higher wet boiling point. Does not eat paint like the others.
I've used both the DOT 4 and DOT 5 on the track with my electric power brakes and have experienced no issues on the track with either. If you have power brakes, I do not believe you will notice a difference. Here are a few links for reference:
http://members.rennlist.com/911pcars/brakefluid.htm
http://www.afcoracing.com/tech_pages/fluid.shtml
http://www.trackpedia.com/wiki/Brake...enced_material
HTH,
Tim