1. Put the oil grade as specified in your user manual / or specified by your engine builder.
2. The number before the W (for winter, not weight - damn fast and furious) is your cold start oil type. Once the engine is up to temp it doesn't have much to do with the operation of the engine/oil. If your car is burning oil when running, changing from a 5W to 10W will do bugger all, you need to look at the last number for a thicker oil.
Oils are essential life blood of your car, don't just go changing things without researching it first.
Good starting point is wiki
Motor oil - Wikipedia, the free encyclopedia