I'm pretty sure Grade 5 will give more strech before failure. Grade 8 just kinda goes like a gun. Thus I'd stay with the 5 cause it will take more abuse 5 is soft 8 is harder......
kinda/sorta/but not really.
Yes, a grade 5 will stretch more before it fails. HOWEVER, that grade 5 has long since snapped long before the grade 8 begins to think about stretching.
grade 5 failure point hasn't begun to hit grade 8 yield point.
About the only place where grade 5 would fair better is under impact loads.
Now, you can get bolts that will bend like a grade 5, yet are stronger than grade 8, but they're $$$$$$.