I've owned and driven a large number of Crown Vic's, mostly cop cars.
The general consensus is that 5w20 is the best oil for those motors. Oil that's too thick causes the timing chain and guides to wear prematurely.

I don't think using 5w40 would cause it to explode immediately, but Ford designed the motor for 5w20 and that's what I always used- even on ex police vehicles with over 225,000 miles, the Motorcraft 5w20 worked great. I can't remember who makes the oil, perhaps Kendall?

Crownvic.net is a weird place.