The “W” following the first number in 15W-50 stands for winter, not weight, and is a measure of the fluid’s flow rate at a seriously low temperature of -15 to -20 degrees Fahrenheit. So at sub-zero temps, 15W-50 will flow no slower than a 15-weight oil. This cold-weather behavior is critical to cranking speed and how readily the oil will flow during initial start-up in cold climates, though obviously it’s more relevant to automobile drivers than to motorcyclists. After all, who’s crazy enough to go for a ride when it’s below zero out? The second number in 15W-50 represents the oil’s high-temperature viscosity, as measured at a temperature of 212 degrees Fahrenheit. For 15W-50, that means the oil will be no thinner than a 50-weight oil at operating temperature.