[Stocker]

It should be somewhat hard to turn by hand, gooey feeling as if it's turning in molasses. Wish I could find a good website to explain it better...

Some use a temperature-sensative silicon fluid which gets thicker (more viscous) as it gets hotter, causing the clutch to engage and turn the fan. Some have an external bimetal spring that acts on the clutch, though I'm unsure if they also have fluid inside.

This design allows use of a very aggressive fan, capable of pulling a lot of air when necessary - but it can turn much more slowly (not actually free-wheeling) when additional cooling is not required, thus saving fuel, robbing less power, and making less noise.

Back to yours - if it turns hard by hand, and sounds like it's pulling a lot of air, it's likely doing its job. Since you don't run hot on the highway, your radiator, thermostat, and hoses should be good. Do you have a fan shroud? If not, the fan isn't pulling much air through the raditor in traffic, though at speed, enough will come through the grill to keep things cool.