the wider the range of the numbers, the thicker the oil. For instance, 15-50 is much thicker than 10-40, which is thicker than 10-30. the W on, lets say 10W-30, denotes winter usage (useless in m'sia).
The reason where there's a range is because those oil are multigrade viscosity. Long time ago, they had single grade oil, like 5W, 10, 30W... well you get my meaning. So a wider range gives you a wider operating temperature range, but it's not as smooth. Now i think only airplanes and heavy industries use those single grade oils.
Synthetics are smoother than organics/minerals, with higher shearing tolerance than the latter.
I'm just spitting out what I learned about motor oil on the internets, heheh. Did some research on it because I had no idea what oil to put in my car last time. Mobil 1 is the best apparently.