Hey I just learned something new today. The W stands for winter not weight.
SAE (Society of Automotive Engineers) Viscosity Grade
Viscosity is a measure of an oil's thickness, or resistance to flow. Lower numbers indicate thinner oil and higher numbers indicate thicker oil. There are two types of motor oils, single grade and multi-grade. Multigrade oil such as a 10W-30, are designed to have the viscosity of an SAE 10W oil at cold temperatures combined with the viscosity of an SAE 30 oil at engine operating temperatures. The "W" or "Winter" designation indicates that the oil meets viscosity requirements for low temperatures (below 30? F).