I understand the reason it's bad is because 5th is such a small gear. Also, fifth is for sustained speeds, not accelerating for a brief time, like the other four gears. Besides, if you top out 4th, you're going way too fast

 

you can boost in 5th gear all you want. that rumor of you cant boost in each gear for over 10 seconds is crap also. The only thing that matters is exhaust gas temp. As long as thats good and your motor is not overheating, your good to go.......Mike

 

concur. more load = more heat = more possibility for detonation/preignition. If the motor is not pinging you aren't going to break it.

I boost in 5th all the time. 12.2:1 A/F ratio egts @ 750C.