My car is a 2002 Altima 2.5 with pretty low miles. I don't drive it too often, and when I do, it's a lot of city driving. Anyway, I'm asking this question from a weather-specific standpoint. It doesn't get too cold here in Los Angeles, maybe 40-45 degrees at the very coldest, and that's rare. Also, I park my car in the garage overnight, so the car's not exposed to those temperatures overnight. However, it gets HOT during the summer (>100). Having said that, I'd like to use Mobil 1, but I'm undecided as to whether I should use 5w30 or 10w30. The dealership uses 10w30 year-round because it doesn't get too cold here. The car manual says that it's okay to use either one at these temperatures, but seems to recommend 5w30 overall. Why doesn't the dealership switch, then? Will I suffer in performance or gas mileage if I switch to 5w30 in the heat of summer? Keep in mind that here, "winter" is only maybe 3 months long. The rest of the year is at least 60 degrees, usually more. Thanks.