motor oil

Use whatever your owner's manual recommends.

Which is probably 5w30 or 10w30

I'd say in the warmer climate of CA it should be fine.

Is that synthetic? You have to watch the non-synthetics on too much spread in the viscosity numbers, as the extra additives that they have to use to achieve that don't help lubrication (dilute the positive effects of the base components that do the lubricating) and are not stable over time (i.e., over a longer oil change interval). I generally go a tick thicker than the manufacturer recommendations anyway as I know their recommendations are skewed towards the lower end in favor of mpg (CAFE) numbers at the minor expense of engine longevity.

I'm in VA, and mix 10W-30 and 20W-50 (3-to-2 winter, 2-to-3 summer) to get approx. 15W-40 on my cars that 30W is recommended for.

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

You should be using a 0W40 synthetic that meets MB approval 229.3 or MB approval 229.5