Oil is an important part of keeping an motor vehicle running. And, basics of oil have evolved over the years. A key measure is “viscosity” which indicates the thickness of oil at various temperatures. The “thickness” factor refers to how fast the oil will flow at certain temperatures. Viscosity is commonly referred to as “weight”.
For reference, the higher numbers mean thicker/slower flowing oil. In short, a 5W-30 oil flows faster than a 10W-40 oil. Generally, using a faster flowing oil (5W-30) in colder temperatures is desirable because more oil flows to internal engine parts for critical lubrication. This provides faster starting and reduced wear and tear on your engine.