Manual says 5w-30, as does filler neck, but manual also says 10w-30 is acceptable if temp never gets below 0 degrees F.

I live in Seattle, and we never get below 0 F here, so is 10w-30 better to use than the thinner 5w in my location. Dealer says its better for our area.

Thoughts on this?

The 5W will be a little less viscous on cold start. Cold start = anything less than your thermostat setting. So, a 5W should be better for you no matter where you live. Once the oil gets to 100C then their both 30 Wts. Check their specks, there's a wide reange in viscosity even in the 30wt range. Compare the oils you are considering and get the 5W with the highest 30 weight visc. at 100C and you should be happy.

DEWFPO

Another thing to consider is mineral oil vs synthetic oil. Some of 10w-30 synthetic oils flow as well as 5w-30 mineral oils. The 10w-30 oils in general shear down less than the 5w-30 oils.
By shearing down, the oil becomes a lower grade. Such as a 5w-20 over time.