Have a look in the Communications Equipment section of the site. Comms Equipment - 4x4 Earth
There are some threads there discussing this topic. Generally, channel 40 is the traffic channel when out on the roads etc. Channel 5 is reserved and can only be used in limited circumstances. A lot of log trucks, around Victoria anyway, use channel 18.
Personally, I use 27 when out bush to communicate with others I am traveling with, but that is just my choice, you could use almost any channel.