I can't see it getting cold enough in California to require a synthetic engine oil. 15w-40 should be more than adequate. If you are worried about the occasional cold snap, an oil pan heater is another option.
It depends on where in California.
Yup - just ask the Donner party ...
Although, 15w-40 is still OK for occasional cold starts, for sure. Typically it's fine down to 20 degF with no real issues. This is a matter of not only the cold temps, but the frequency of those expected temps. If you only occasionally get down to 20F or so, then 15w-40 is fine year round. If it's 20F all winter long then a thinner 10w-30 might be better.
And ... it's not really about wear rates and those cold temps. I've got a bazillion UOAs that show wear is not greatly affected by some cold starts. What REALLY matters in a diesel is the cold-cranking ability of the rotating assembly. When it's really cold outside, that parasitic drag of thicker oil make make just that slight difference in spinning the engine not quite fast enough to get enough compression heat for ignition. In older IDI diesels, this was most certainly a concern. In modern CRD injected engines with intake heaters, it's not quite as much of an issue.
I've seen plenty of data to show that either 15w-40 and 10w-30 will work just fine year round in most any environment and service factor.