The reason I changed them was when I was installing the hitch, I could not get the stock bolts to sinch at 80 ft lbs. I pulled the bolts out and found that 1 was stretching. The new grade 8 bolts tightened up perfectly. (yes, I have a fresh, well maintained torque wrench)
This is the 3rd FJ hitch I have installed and had no issues with the 2 before.
DH