[LX5liter]

The W after the first # is the "winter" weight, which is the viscosity of the oil in colder temps (there is a certain temperature they use as a benchmark, but I forget what it is). The second # is the "hot " weight, which is what the viscosity is at higher temps. Viscosity simply means "resistance to flow" and the higher the #, the thicker the oil. Two #'s together (10w-40) means a multi-grade oil. A single# (50w) is a straight grade oil which means its viscosity does not change when hot.