The fluid inside gets thicker as the temp gets hotter. So yes, at higher ambient temps, the fan will pull harder (or slip less, however you want to say it) and pull more air through the grill. Over time, I have heard that the fans can develop minute cracks that slowly allow the fluid to seep out thus eventually causing failure.
On a really hot day at operating temp, the thing should be able to take your hand off, so don't stick it in the way. However, when you first start up on a cool am, you should be able to hold the fan from spinning at all.
Best to tests when engine is off. If really hot, you will not be able to spin it very easily. It will be very free when cooler.