Engine oil viscosity denotes how easy an oil will pour at a given temperature. Lower viscosity oils, such as 5W-30’s, are thinner and well-suited to colder weather. On the other hand, higher viscosity oil is thicker and is better at maintaining performance under high load and heat. Essentially, it’s how “runny” the oil is, and the viscosity that’s best for your engine depends on any driver’s manual recommendations and the overall climate you’re in.