Motor Oils

Oil lubricates and cools engine parts and keeps your engine clean. The oil is drawn through a pickup tube and screen from the oil pan and circulated throughout the engine under pressure by the oil pump. The oil is filtered and then sent to the engine bearings and valvetrain. Splash from the connecting rods, and on some engines jets of oil, lubricate the cylinder walls and cool the underside of the pistons. Oil then returns through passages to the oil pan.

There are two kinds of engine oil, conventional petroleum oil and synthetic. Conventional oil is refined from crude oil pumped out of the ground, while synthetic oil is chemically constructed in the laboratory from various raw materials. Synthetic oil is superior to conventional oil at temperature extremes because it flows better at cold temperatures for better startup protection and resists high temperatures that can cause ordinary oil to breakdown.

All oils have additive packages. Detergents keep varnish and sludge from forming while dispersants keep harmful materials suspended so they can be drained out. Extreme pressure additives and friction modifiers reduce wear for longer engine life and reduce friction for better performance and mileage. Viscosity improvers minimize viscosity change with temperature. Other additives reduce foaming and oxidation, neutralize acids and combustion byproducts, and prevent rust and corrosion. The additive package in synthetic oil is generally superior to that of conventional oil.

Oils are classified by grade and standard. The grade is a numerical index established by the Society of Automotive Engineers (SAE) that represents the oil’s viscosity, or resistance to flow, commonly referred to as “weight”. In this system a 10 weight oil flows easier than a 50 weight oil at the same temperature. Most vehicle manufacturers recommend a multi-grade oil like 10W-30 (the w stands for winter, not weight) because it will flow easily like a 10 weight oil under cold temperatures, but have the characteristics of a 30 weight oil at 212°F. It is best to use the grade of oil recommended by the vehicle manufacturer.

The American Petroleum Institute (API) evaluates lubricants against minimum performance standards, with service classes for gasoline and diesel engines. The gasoline engine standard begins with “S”, while the diesel engine standard begins with “C”. The current gas engine standard is SN, with CJ-4, CI-4, and CI-4 PLUS current for diesel engines. The standard and grade can be found in the API service symbol, or “Donut”, on the oil container. Some oils may also have the API “Starburst” symbol, indicating the oil meets fuel economy standards set by the International Lubricant Specification Advisory Committee (ILSAC).