Grade 5 for a shear load and grade 8 for a tincile load. Yes it is true a grade 8 is stonger than a grade 5 but they are also more britle. I learned this from a fence posthole awger. I was using grade 8's for the shaft pin and they would shear right away. An old former looked at one of my broken bolts and told me to use a grade 5 insted. Its softer and will flex where the grade 8 will snap. He was right the same grade 5 bolt is in there today hundreds of holes later.
