Explained simply multi-grade motor oils have a viscosity rating made up of 2 numbers, indicating that the oil has different flow characteristics when cold from when it is hot.
The viscosity rating of a multi-grade oil is broken down into two parts:
The number before the W is the "winter" or "cold start" rating. The lower the number the thinner the oil. This allows quick flow when the engine is started cold (cold start). 90% of engine wear happens at this time due to the engine running in a dry state.
The second number is the viscosity rating of the oil at 100 C. The higher the number the thicker the oil. If the oil is too thick for the temperatures your engine is running at, the oil will not circulate freely. If it's too thin, it may not lubricate effectively.
As you live in a warm climate I wouldn't go below 5w and no higher than 40 on the thickness scale, BMW recommend LL 04 as their preferred choice I personally use Mobil 1 ESP 5w-30 LL 04 fully synthetic oil in my Z without any problems, the above information I quoted is freely available from the Opie Oils technical UK web page.
If it was my choice even living in a warm climate I would stick with a good quality fully synthetic 5w-30 BMW LL 04 approved oil.
Hope this helps.
