1. most brands are fine to run in your car. Just stick with one though. I like Trinity Monster Horespower. O'donnel is also good. 20% is the pretty standard amount of nitro for regular use.
2. I really don't recomend the onboard temp gauge! I had one and it worked horibly. It even failed the boiling water test. Instead get a pro exotics temp gun at www.tempgun.com
I just got one and I love it. Its only $35 plus $5 shipping. Very small too. probably 1in. x 2in. Very accurate like raytek guns. Completely worth it, because temperature is an extremely important thing when your tuning your engine.
3. same as he said below
4. don't know, probably not very common if there is one.
hope this helps