Metalman,
The link Lawrence provided is a great tool, but also try to remember why those numbers are there. For example, a grade 8 can hold more than a grade 5 because it is stiffer. Now, I'm not sure if the alloying elements are changed, but even if they are, I think that this stiffening does not raise tensile by as much as it raises yield, and therefore yield is now considerably closer to tensile. Basically, while a grade 5 would stretch considerably if put under extreme load, an 8 would break at about the same time it started to stretch (albeit at a higher internal stress). Perhaps your torque specs are taking it to this point (I noticed 131 ftlbs for a 1/2 inch on the sheet), and a well lubricated bolt does not read near as much torque as a non lubricated one, and, bam, all the torque goes into stretching the bolt, resulting in a grade 8 to fail while a 5 would have had that sluggish feel to it by that time that would tell you that it was about to break.
These differences would be similar to a rubber strap and a piece of glass. The rubber is going to stretch and you will be able to notice it stretching a little too far, but the glass will hold until it shatters without warning.
Also, remember that those numbers on that page are for tension. If you are cantilevering using bolts as the shear points, you have to factor that into the overall stress, which can get quite large.
I realize this is more than you actually asked for, but I hope it helps anyway...
gls
With the torquing of any bolt, torque does not necessarily equate to clamping force. You can get high torque value if the bolts and nuts have been coated or galvanized, the class of threads, the thread surface roughness and whether the bolts and nuts have been lubricated. I assume this is a structural type of application, do you have washers under the turned element. High strength bolts like a grade 8 can be much harder that the base metal you are connecting and the turned element (nut or bolt) can actuall start milling the surface increasing the torque value you are obtaining without increasing clamping force.
There are devices that can measure clamping force and you can determine a more precise torque value for the actual bolt and nut combination that you are using by test your combination in such a device.