Now day you can not drive more than 150 km without seing a petrol station.
I drove once to Salalah and i have to re-fill at any first petrol station i see.(To be on a safe side)
So you may not need any additional Jerry Cans or sub tank. (This is my Opinion)

There seems to be a lot of info on sub tanks at www.patrol4x4.com (according to Google searches) and also here: http://www.exploroz.com/Forum/Default.a ... y=sub+tank and possibly here: http://www.nissan4x4.co.za/forum/default.asp . However, you have to register to see it properly (and my self respect prevents me signing up to a Nissan site! )