There is a technically a difference in viscosity at start up between 5 and 10, but in reality the difference in the flow profiles between the viscosities for Australian temperatures is insignificant to warrant much debate.

So to keep it simple, Xw-Y

X is the viscosity at start up - COLD
Y is viscosity under operating temperature (measured at 100 deg)

So unless you live in freezing cold climates or rev the crap out of your car when you start it up, you can run between 5-15w with nil problems - it's only valid for initial start up.

Once the engine is warm or at operating temp, value Y is the important one - higher Y = more viscous. If it's too low, it's too thin and if you're racing or something... you might not get enough lubrication leading to wear. Having a heavier oil is better for lubrication, but I believe it may impinge on fuel consumption and it may damage the oil pump as more viscous oil requires more pressure to push through thus again, consuming more power to pump.