I just checked the manual/parts list for my woods 4" mower. The manual covers the larger models too.
In BIG BOLD type is says to use only grade 2 bolts for the shear bolts.
My snow blower also calls for grade 2.
I strongly suggest NOT using grade 5 or higher. The softer bolts are used on purpose - so they shear under stress. I blow out one or two every time I use my snow blower because I keep picking up rocks. When I first blew one out, I didn't have a grade 2 replacement so I used a grade 5. Next rock I caught blew out the #50 drive chain. Now I only use grade 2 and I buy them a half dozen at a time.
If you find you are breaking them frequently, as I am, switch to a slip clutch. That's what I plan to do this summer. Changing the shear bolts is getting old - but it still beats replacing the gear box or chain.
Peter