Multi-viscosity oils, whether conventional or synthetic or gas or diesel have a higher viscosity index, meaning that they act like a thin oil (the smaller number i.e 5W) when cold and a thick oil (the larger number) when hot, instead of thick when cold and thin when hot as a single viscosity oil would. Synthetics do have a wider range of viscosity than conventional oils. I wouldn't be afraid of running a full synthetic year round, but I'm a cheap bastard and don't want to spend the money on synthetic when I'm going to change it on a regular basis anyway. Some guys run synthetic for longer intervals, but I like to drain the abrasive particles that it picks up on a regular basis rather than let them circulate around the engine.
