Many drivers think the W in 5W-30 means weight; in oil history it actually marked "winter." Early engines ran single‑grade oils and owners swapped seasonally, until viscosity modifiers allowed one oil to flow in cold and still protect when hot. Those polymers uncoil as temperature rises, tempering viscosity loss, while detergents and dispersants keep deposits suspended for filters to catch. In Nordic use, this reduced seasonal changes and made cold starts gentler without sacrificing stability on long drives. A modern example is MOTOR OIL XT 5W-30, built on that principle.