couple of things here. first off your truck DOES NOT have a "low oil level warning" device!
what it DOES have is a "low oil presure warning device"! it works like this: when the oil pressure gets low becuase of lack of oil in the pan or a faulty pump, the oil pressure will drop, if it drops below the level of the oil pressure sending unit's operating range the guage will drop and at the same time turn on the oil light as a warning.
You are correct the low oil pressure sending unit is basicily an all or nothing unit, anything above 6-10 psi "it's happy" and sends a voltage to the pcm and your guage to put them in the normal range.
The fact that your truck (4.6L especially) can be really picky about what oil you use, my bro has that motor in a Lincoln and will burn a quart to 1000 miles (no smoke) just goes away. He switched to fords oil and weight and never uses a drop (go figure) On mine I use Mobile One and never use a drop. "Just for what it's worth"
Now some engines have more oil capacity in there pans than others. So when the oil level gets low and you brake to come to a stop, all the oil can be forced to the front of the pan for a few seconds (depending on your braking period) and the oil pump gets starved for oil and will not produce pressure and turn on the warning light and kill the guage, this should return after the pump picks up oil again, a second or two after your stop.
If this is whats happening! Of coarse the best thing to do is get it to were it's doesn't use that quart of oil.
Hope this has been informative and not too long.