Mechanical fan clutches are fun!
After sitting all night the silicone fluid leaks out of the reservoir, so when the engine is started cold the fan clutch will engage until the fluid is "spun" back into the reservoir. When not engaged the fan clutch spins at 30 percent of shaft speed.
When the air passing through the radiator and hitting the bi-metal spring on the front of the fan clutch is at about 170 degrees (coolant in the engine will be at about 200 degrees), the bi-metal spring will slowly begin opening the valve and the silicone fluid will come out and start to engage the fan clutch. When fully engaged the standard fan clutch will turn at about 60~70 percent of shaft speed (an HD fan clutch will give you 80~90 percent of shaft speed).
How do you check a fan clutch? Well, there is a procedure in the FSM. A quick and dirty test is to first thing in the morning, before starting the engine, try and spin the fan clutch by hand a couple of times--mentally note the resistance felt. Then start the engine and let it run about 5 minutes, shut it off, and after the fan stops turning (easier on the fingers that way), try and spin the fan by hand again--the resistance felt should be less because all of the silicone fluid has been spun back into the reservoir and the bi-metal spring is keeping the valve closed. Ok, now restart the engine and drive it for 20 minutes to completely warm the engine, and make the last few minutes just driving around town. Shut the engine off, when the fan stops try again to spin it by hand--it should be STIFFER, if it isn't it should be replaced.