Since this gets ask so much just want to break it down to it's simplest form. You should be running oil based on your climate if you look in your owners manual you will see a chart what what type of oil you should use in what climate. But here is the simplest break down I can give.

The recommended temperature range are as follows

10w30: -13 F to 100 F (-25Celsius to 38Celsius)
10w40: -13 F to no limit (-25Celsius to no limit)

In other words the difference between the two oils if you are hesitating in switching over is that, the 10w40 has same performance at low temperatures, but works well also above 38C/100F. Your vehicle will experience above 38C in Manlia trafffic meaning 30 weight oil will break down quicker and provide less protecton. But if you live someplace like Baguio you can run 30 weight oil year around.