Time for a quick refresher: in multigrade oils, there are two viscosity ratings. For example 5W40 indicates an oil which is thinner when cold and thicker when hot. The "W" indicates "winter" - more typically "cold". So 5W is an oil which has a viscosity value of 5 when cold. The other number is the viscosity when hot. In this example, a viscosity value of 40. So why use a multigrade oil? Well when the engine is cold, you want the oil to flow as quickly as possible to all the engine parts, and also not to impede the power of the starter motor in turning the engine over. But if the engine oil stays thin when it gets hot, then it does no good because it runs too quickly, and doesn't stick to any moving parts. Hence multigrade oil that thickens up with temperature.
So which oil should you use? Suffice to say that a 5W oil isn't going to give you much benefit if you live in Singapore in the same way that a 20W oil won't do you any good if you live in northern Alaska. Here's your rough guide:
5W-30 Cooler climates, like Sweden or Canada
10W-40 Temperate climates, like England, northern Europe, New Zealand
15W-50 Hot climates, like Italy, Spain, Egypt, Australia