Refer to my charts, and be sure to consider the operating environment...
High heat, and "annealing cycles" of heat and gradual cooling - both work against the original heat treatment of the metal. Also, the heat and dry will work to gall the screws to the threaded holes, making them difficult to remove.
With that in mind, use brass or bronze if you can get them. The installed tension is not enough to cause a problem with the change of material, and copper-based alloys won't seize on iron or steel. Also, copper-based alloys respond to heat differently than iron-based alloys - they'll get incrementally harder. Fastenal carries silicon bronze screws - and they work well.
If you can't get brass or bronze screws, then get SAE Grade 5. The SAE Grade 8 will lose strength much faster than the 5's (they're more sensitive to annealing. We experimented with this in "Materials and Processes" last year...) and you can get SAE5 screws pretty much anywhere.
However, brass or bronze is preferred for exhaust hardware - manifold screws, flange stud nuts, muffler clamp nuts - if you can replace it with brass or bronze, do so. You'll thank me next time you take it apart...
