The reason for the requirement of ULSD is the particulate trap. Excess sulphur will destroy them, just as tetraethyl of lead would destroy a catalytic converter on a gasser. If your truck is not equipped with a particulate trap (5. 9 liter) you can run regular diesel and not worry about it. If you have the particulate trap, (6. 7 liter) and you run the higher sulphur fuel you'll quickly (as few as one or two tanksful) destroy a very expensive piece of required emissions equipment, without which you'll never be able to pass a smog test at the requisite levels for your vehicle. Also, once it's plugged I doubt the exhaust will flow very freely at all so your performance would probably go in the toilet anyway, even if the computer controls didn't put you into "limp mode" because of the excess back pressure, which it almost assuredly would. Simply put, if you have a 6. 7 and fail to run the ULSD, you'll very shortly wish you had.