From: Winnipeg, MB
The reason it sounds "intermittent" is that tankers are too valuable to send in an unescorted or poorly escorted small convoy. Better to set plan a big convoy with over 100K fuel once a month or so (or embed the TK/AOs in a troop convoy to use their escorts). It takes about a month to get to Australia from the USA anyway.
For Australia I have even hauled from Abadan to Perth, if the IJN is absent from the IO. Others haul fuel from EC USA to CT, then on-map to Adelaide or Melbourne. As long as you dump fuel at a port with a rail connection to the industrial cities it will go where it is needed.
As for hauling Oil instead of fuel, it is the same situation as resources vs. supply. IIRC it takes 10 oil to produce 1 fuel so if you want a place to get fuel, bring fuel. In the case of the Japanese Islands, Hokkaido has some oil but needs fuel for ships to use the port there so hauling fuel to Hokkaido or Sakhalin and bringing oil back is useful.
For Australia, at game start haul fuel from the DEI and if you exhaust that, then haul oil if you can. Use small tankers only in this dangerous area. You don't want a big one sitting for days loading.
No matter how bad a situation is, you can always make it worse. - Chris Hadfield : An Astronaut's Guide To Life On Earth