The first number is the COLD viscosity, the second is the HOT viscosity. The oils have additives to change the viscosity with changing temperature.
Its debatable how low you need the cold viscosity. a low number means that the oil will be circulated quickly from a cold startup. But a higher number (thicker when cold) is likely to hang around better, so not need the ability to quickly circulate when cold.
I think the quick circulation factor usually wins out.
A 5 is really the lowest you [b:adb267e0b1]need[/b:adb267e0b1] to go in Oz, unless you are up at Perisher or something (then you might need a -5).
Probably what's more important is the API specification (like SG/CF or something like that) I think the latest is something like SJ/CF (anyone?), and indicates the level of protection that the oil gives. The 'no brand' oils (KMart, etc) are often only SG or SH API specs, so steer clear of them.
