Ford's Super Duty 6spd in "M" manual mode will hold 1, 2, or 3 for as long as you'd like from a dead stop to bumping on the limiter and won't shift until you bump the "+/-" button on the shifter stalk, lugging or screaming makes no difference. In 4, 5, or 6, it will hold them up to the limiter if you're running hard, but, it will downshift past any/all of them if you lug too far. Case in point, you're running down the highway in M/6 and forget to downshift coming up the off ramp. If you have to start from a dead stop and do nothing, it downshifts to M/2 on it's own, then it'll hold M/2 until you start working through the gears on the "+/-" button again.
