The recommended fuel in my truck manual is 91
research octane, which in the US is not the number that appears on the pump. The number on the pump is an average of research and 'motor' octane. See these two links:
Octane Explained
Octane rating - Wikipedia, the free encyclopedia
Whether that's regular, mid-grade, or premium, or some other 'grade' depends entirely where you are in the country.
As for what happens if you run lower than recommended? That's debatable. You're theoretically robbed of power and efficiency (mileage), though running a higher octane doesn't generally improve either. Also, the truck has a computer controlled engine and will make timing adjustments to compensate if you're running lower, so you may or may not hear knocking. If you do, that means it's too low to compensate for. And if you switch fuels, expect it to take at least a couple of tanks to adjust itself. You can speed that up by disconnecting the battery (resetting everything).