The temperature range the oil is exposed to in most vehicles, however, is too wide for single-grade oils, ranging from cold temperatures in the winter when the vehicle starts up to hot operating temperatures in summer weather. That's where multi-grade oils come in: by nature the oil will have a high viscosity when cold and a lower viscosity at the engine's operating temperature. The problem here is that during cold-starts the thick oil needs longer to get to the necessary engine parts and do its job, whereas at higher operating temperatures it might get too thin and the coating film could break in some places. To bring the difference in viscosities closer together, special polymer additives called viscosity index improvers (VIIs) are added to the oil. These additives are used to make the oil a multi-grade motor oil (e.g. 10W-40). The idea is to cause the multi-grade oil to have the viscosity of the base grade when cold and the viscosity of the second grade when hot. This enables one type of oil to be generally used all year. In fact, when multi-grades were initially developed, they were frequently described as 'all-season oil'.