If, as I have read from multiple sources, DOT 4 and DOT 3 fluid is interchangeable, why would a vehicle specify 'DOT 3 ONLY'? Is it a materials-compatibility thing or, since this is a GM truck with the stupid intentionally-squidgy pedal feel, simply part of the idiot engineers trying to make sure the pedal is nice and soft to keep the idiot soccer moms happy? Will anything dissolve if I use DOT 4 (or 5.1)?