One of the big difference between grade 5 and 8 is the elasticity of the bolt.
Grade 5 has a larger range of stretch VS grade 8. Grade 8 has a smaller range of stretch and shrink. So they will hold better but will snap easier if run out side the designed spec. While a case hardened bolt is harder to break but has less stretch then the grade 8 and is good for things like frame to spring hanger connections but bad for things that get hot and cool off. Stainless is good for things that corrosion is a problem like the header.