I would be hesitant to use anything other than what Cat recommends. While a Grade 8 bolt is stronger than say a Grade 5 bolt, generally the Grade 8 bolt will be less ductile and depending on the loading of the bolt, can actually be a poorer choice than a Grade 5.
Depends on the application
The threads root cause an extreme Stress concentration and in this application if the bolt were to be subject to bending from loading multiple times, the thread root could work hardened, crack, cause a higher stress concentration then fail.
Before you run out to a local bolt store you also need to know if the original bolt has a rolled thread as compared to a cut thread.
Very few bolt stores will stock a rolled thread bolt
When higher strengths are needed, often manufacturers will often employ a rolled thread bolt which increases the overall load that the bolt can carry and can reduce fatigue issues.
Buy Cat and hopefully the engineers have figured out the best bolt characteristics for the application.
An aside, to determine if a bolt has a rolled thread or cut, just run your fingers down the bolt from the shoulder to the thread, if the thread has a slightly higher diameter, NORMALLY, it is a rolled thread.
