The viscosity rating of a multi-grade oil is broken down into two parts;
The number before the W is the "winter" or "cold start" rating. The lower the number the thinner the oil. This allows quick flow when the engine is started cold (cold crank). 90% of engine wear happens at this time due to the engine running in a dry state.
The second number is the viscosity rating of the oil at 100 C. The higher the number the thicker the oil. If the oil is too thick for the temperatures your engine is running at, the oil will not circulate freely. If it's too thin, it may not lubricate effectively. Take a look at our great range of Stroke Engine Oil from Opie Oils