1-Is it bad to "Down shift" in a high speed or let car stay in that gear and brake?
2-Is it good to red line once while to keep the car running good?
3-Is it good to wash the engine bay?
4-Do I and need to change Transmission flue for a manual

 

1.) So long as you RPM match or don't over-rev when you down shift you should be fine
2.) Redlining your engine once and a while won't hurt your engine. Just don't make it a habit.
3.) I would say that it shouldn't be a problem. You would definately see steam when you sprayed the EM. I would definately recommend to wash it while the engine is warm and to dry it by letting the car idle. And I would recommend to not wash it while the car is running.
4.) All fluids required to be changed. Timeframes are different for each type. Consult your manual or Fluid manufacturer for recommend fluid lifecycles.

 

1. There shouldnt be any harm if youre able to do it right. But as many state before, brakes are a lot cheaper than a clutch.
2. Yes
3. I have known ppl that have 10 year old cars and never washed the engine bay once. I dont think there is any harm in washing. I dont wash it when the engine is warm, it's usually cold. And I dont let it dry by idling the car. Just bag the filter if youre using a SRI.
4. Yes, but not as often as an auto.

 

i usually hose down the engine bay and quickly dry it off with a leaf blower . done this everytime i wash the car for 3 years now. never had any problems.