Oil weight or the viscosity rating is essentially the thickness of the engine oil. Most modern engine oil has a multi-grade rating; the number with 'W' represents its thickness at 0 degrees Fahrenheit, and the latter number represents its thickness at 212 degrees Fahrenheit, which is the engine's operating temperature. The larger the number, the thicker the oil is, and vice versa. The key thing is to always use oil within the weight range specified by your car's manufacturer.