The viscosity of the oil for any application depends on the speed, load, and the clearance between moving parts, type of lubrication and finally the ambient temperatures. Oil must provide protection over a wide range of temperatures from cold starts to long hauls. When oil does well at both extremes it gets a "multi-grade" rating that includes a number/letter followed by W and a number, such as 20W40. In a hot climate or driving through a region where a combination of high heat and high-altitude are encountered, the operation & maintenance manual provided by the manufacturer, will probably give suggestions for a multi-grade or "all-season" oil such as 15W-40. Such oil has the cold flow characteristics of 15-weight at start-up, yet the coating and protecting property of 40-weight oil at the high temperatures. In a predominantly cold climate, the recommended oil might be a 5W-30. However it is advisable to be guided as given in the operation & maintenance manual of that equipment.

Lubricant is a substance interposed between two surfaces in relative motion for the purpose of reducing the friction and wear between them. Lubricant provides a protective film which allows for two touching surfaces to be separated and "smoothed," thus lessening the friction between them and correspondingly less heat generation.