multi-grade oil - engine oil that meets the requirements of more than one SAE viscosity grade classification, and may therefore be suitable for use over a wider temperature range than a single-grade oil. Multi-grade oils have two viscosity grade numbers indicating their lowest and highest classification, e.g., SAE lOW-40. The lower grade number indicates the relative fluidity of the oil in cold weather for easy starting and immediate oil flow. The higher grade number indicates the relative viscosity of the oil at high operating temperatures for adequate wear protection. The "W" means "winter" grade. Multi-grade oils generally contain viscosity index (V.I.) improvers that reduce the tendency of an oil to lose viscosity, or thin out, at high temperatures. See SAE engine oil viscosity classification.