Toyota's recomendations in regards to oil are stupid. They make no account for weather, driving habits, or engine age. You can't hurt you car be putting in a different weight. Just make sure you change it regularly.
10w-30 is pretty much the "catch all" oil that is put in if you don't specify a certain weight to the guys that do it (almost anywhere in CA). I live in California too, and it seems to be fine for me (I use Mobil 1 fully synthetic 10w-30).
5w-30 is probably a bit too light. Espcially if you do hard driving. I doubt you'd notice any difference (except maybe a VERY minute improvement in gas mileage). Some people I know drive with even thicker oils (particularly when the piston rings really start to deteriorate). In fact, a former Toyota engineer I know says he gets the best performance out of 20w-50. YMMV.
- John