Simple, Just use Shell Rimula-X 15w40 diesel oil in your quad and also every 4 stroke racing bike, car, truck, outboard, bull dozer, mower etc that you own. Seriously. In Perth or Sydney typical air temps you wont go wrong with this oil. It is cheap at $60 for 10L, and it is so much too good for your quad you can sleep easy.
You can think of the 15W as the thickness when the oil is cold. The 40 is for when the oil is up to normal engine temp. Unless you get below 10C often you can pretty much forget about the number before the W. Except if you don't change your oil often enough, then it can/will break down to the viscosity of the number before the W! The number after the W you match to your typical air temps and the type of engine. Typically 40 or 50. 30 is mostly a joke IMHO.