How often does the coolant need to be flushed?

Both my dealer and Indie shop kinda look at me me funny anytime I inquire. Either it's a real PITA for them, or it's not as important as I may think?

 

Because porsche claims its "lifetime" coolant and doesnt need to be changed unless there is an issue like adding the wrong coolant or oil/coolant mixing issue.

 

The jack-the-back-up method doesn't work... I had it done twice like that, and still had air-bubbles in the system. Lots of overheating this past week during hot weather and stop and go traffic. Finally had to take it to a (better) indy mechanic today, who could re-flush the system with a vacuum pump.

 

What happens if you use tap water, specifically Houston, TX tap water. :0