If the existing oil in the engine is a detergent, drain it and install a good brand of new 20-50; ideal for the hot valley. I personally use Casterol GTX 20-50, started using it when I lived in Sacramento.
Caution !!!! If the existing engine oil happens to be non-detergent, suggest you remove the pan and clean out the sluge that has accumulated in the bottom of the pan. If you install a detergent oil without cleaning out the pan, the sluge will be picked up and distributed into the valve chamber and could plug some of the passage ways to the main bearings.
Ron