"When replacing fasteners, it is generally best to match what you are replacing. Replacing a bolt with a stronger one is not always safe. Harder bolts tend to be more brittle and may fail in specific applications. Also some equipment is designed so that the bolts will fail before more expensive or critical items are damaged."
"Grade 5 / Grade F
Grade 5 bolts are hardened to increase strength and are the most common bolts found in automotive applications. Grade 5 bolts have 3 evenly spaced radial lines on the head.
Grade F is roughly equivalent to Grade 5. Grade F nuts are used with Grade 5 bolts.
Grade 8 / Grade G
Grade 8 bolts have been hardened more than grade 5 bolts. Thus they are stronger and are used in demanding applications such as automotive suspensions. Grade 8 bolts have 6 evenly spaced radial lines on the head.
Grade G is roughly equivalent to Grade 8. Grade G nuts are used with Grade 8 bolts."
Above are a couple of excerpts on the bolt grades and their uses. I always tend to go with the strongest one available. The 2nd excerpt seems to indicate that 8 is used in more demanding situation. The 1st would seem to indicate that Blue Ox is either trying to match the quality/grade of the other components in the hitch or want the shank bolts to fail first?, which I can't imagine.
From your experience with the apparently stripped bolts/nuts and then the bolts snapping in half I can only say that I would upgrade to the grade 8 and also ask for their rationale on discouraging the use of the stronger bolt.
Danny & Susan wife of 53 years
2019 Ram 3500 Laramie CC SB 6.4 4x4 4.10
2020 Montana High Country 331RL