jsg,
As others have been hinting, you should ensure that you are not leaking. Apart from that, you should take current ambient temperatures into consideration. I believe it is currently summer for Australians at this time of the year, but personally, I don't know how hot it gets there.
If you haven't done so yet, you might want to consider a 5W50, 10W50, 5W60, 10W60 formulation instead.
And, you should also provide us with a bit more info such as:
- Driving conditions (ie. short trips, stop-and-go, highway, etc.)
- Driving habits
- And of course, weather temperatures.
Regards,
Oz