When you're looking to change your motor oil, it's crucial to pay close attention to that W in the oil grade rating. W stands for Winter, which defines how a particular oil's viscosity may change in colder temperatures, which differs from its viscosity in hotter conditions. Let's take 5W30 as an example. The number after the W, which is 30 in this instance, is the rated viscosity of this oil at an engine's operating temperature of around 100 degrees celsius. Meanwhile, the number before the W, which is 5 in this case, is the relative viscosity of this oil when the conditions get colder. This doesn't just mean driving over the winter, but also giving your engine a cold start. Thanks to how modern motor oils are engineered, they can vary in viscosity (i.e. become less viscous) if the temperature drops.