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best thing as nick says is to get what the manual recommends as its made to run with it.
If not, and you want to get all techy, then the best suiting oil depends on the temp range your gonna be runnin the car in.
its best to use a multi grade oil that has teh smallest span of viscosity that can be used in the temps you are gonna be running in.
so oils like 10w-40 and 5w-30 have loads of polymers, which means that it is more prone to thermal & viscosity breakdown due to the high content of polymers. The polymers can shear and burn leaving deposits which are bad for ring sticking.
The new mobil 1 15-w50 is very good and its ability to pump at -35 F is as good as most 5w-30 oils.
easiest way of looking at oils is to think of a 15w-50 as a 15 weigfht oil that will thin no more than a 50 weight would at 100 C.
a 5w40 has (if its a multi vis and not a synthetic) ALOT of polymers to control viscosity thinning as temps increase........and lots of polymers is a bad thing.....
and like i said, a 15w-50 MOb 1 has the same pouring characeteristics as most 5w-30s.
anyhow......a 0w40 in aus should be fine, but see if you can get a 0w40 synth as it wont thin out as much under your extreemely hot temps. but there is no need to take oil to the extreeme, as long as its suitable for the engine, just use it.
i wouldnt worry too much, there is far too much detail to go into on oils, such as flash point, pour point, ash % content, zinc content, viscosity index etc etc.....so you can go overboard tyring to find the best oil for the engine. Its easier to use the manufacturers reccomendations as they have already done all that. if you do wanna change, keep the same 2nd viscosity number otherwise youll be running too thin.