Did some research. Grade 8 has greater strength in shear and tension. than grade 5. The difference is the plastic range during elongation until failure. Grade 8 shorter plastic range. But occurs at higher tension loads. If that were the only parameter grade 8 would be superior. Grade 5 is the choice as it is less susceptible to stress corrosion. So it is advised to use the original bolt grade especially in corrosive environments such as suspension.