Napa may have sold you a "severe duty" viscous clutch that locks up as low as 180 degrees.

i know that the factory 4.7 v8 viscous clutch contains an internal silicone fluid that is even more "watery" than the 5.9 v8 and allows the fan to slip more so that it turns at a lower RPM and saves a bit of fuel.

i think the FSM says that the factory stock bimettalic coil inside the clutch that locks it up is set for 220 degrees F,
below that it is supposed to slip and fan blade rpm is supposed to be less than 800
even if the engine is at 1000 to 5000 rpm.