I'd make it a two part question.
As to the 20 vs 30 weight, it's not a very big deal. You'll get probably not noticeable but better mileage on the 20. On some engines it seems the 20 revs more freely, but many people also complain about increased noise and harshness. If it were my car, I would try both and see if it matters to you, or just stick with the 30 since you seem happy with it.
If it's only driven in the summer, it probably doesn't matter if you use 0-30, 5-30 or 10-30. In the Mobil1 universe, you would be trading HighTempHighShear performance for a lower pour-point by going from 10-30 to 0-30, but I haven't taken a look at AmsOil specs and I'm sure someone knowlegable about them will come along soon enough with a better recommendation than I can give.