are engine oils 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 10W-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.