[jdmccright]

The only difference between 5W-30 and 10W-30 is their viscosity when cold. 5W is easier to pump when cold, thus it gets circulated slightly faster. But once it is hot, both will perform the same as a 30-weight viscosity oil.

In your climate, you should not have to worry about lack of lubrication due to slow circulation from cold oil, so you could probably use 10W-30. Indeed, many truck owner's manuals for that era allow the use of 10W-30 for operating environments that stay above 20°F or so. Check your manual.

For older vehicles, the switch to thicker oil by some is typically to reduce the amount that leaks out from seals when the vehicle is sitting and off. Thicker oil drips out more slowly.

Hope this helps!