Thicker oil makes the front stiffer and fork oil should be changed every once in a while. Remember that fork springs start to sag too and a common way of dealing with that, and stiffen them, is to pack the tops with large washers - or sometimes 2p coins.
To measure the air gap - empty the fork. Fill with the correct amount of oil. Get a stick and slide this in the fork, mark where the stick where it touches the top of the fork. Pull the stick out and measure the stick from the top mark to where the oil is on the stick. That is your air gap for future reference.