Oil viscosity changes

Motor oil behaves like honey on a cold winter morning—it becomes thick and flows slowly, making it harder for your engine to start. On a hot summer day, the same oil thins out like water, which could reduce protection for engine parts. This is why most modern oils are "multi-grade," containing special additives that act like tiny coiled springs: they stay compact when cold to let the oil flow, then expand when hot to keep the oil from becoming too thin. A bottle labeled "5W-30" works like a thin oil during cold starts but protects like a thicker oil once your engine warms up.