Oil Container Labels

The American Petroleum Institute (API) logo is a circle divided into three sections. The top half of the circle is the "API Service" section, which is used to indicate the quality of the oil. The lower half of the circle is the "Energy Conservation" section, which indicates the oil's degree of fuel efficiency. The center of the circle is the "Viscosity" section, which is used to indicate the viscosity of the oil. The oil container should have a "Starburst" label instead of the doughnut shaped symbol.
Multigrade Oils - These oils make seasonal changing unnecessary. They contain viscosity-index improvers that enable each oil to exhibit suitable flow properties at both ends of the thermometer.
They have labels like SAE 5W-30 or 10W-30 or 15W-40. Some may have labels like SAE SW-lOW-20W-30, in this case, just ignore the middle numbers and read the oil as SAE 5W-30.

OIL CAN SAVE YOU GAS
Many factors are involved in determining the overall fuel economy obtained when driving a vehicle. One of the factors is the fuel efficiency of the engine oil being used.
In addition to being rated on the basis of their performance and viscosity, oils are rated on their ability to reduce the amount of fuel consumed during driving. Oils are rated against a reference oil in a standard engine test much like the ones described earlier in the "Oil Performance Ratings" section.
In this particular test, the fuel economy is measured and compared with that of a reference oil. If the tested oil provides at least 1.5 percent better economy than the reference oil, it is labeled "Energy Conserving II." These energy conserving designations are indicated on the lower half of the API logo to indicate fuel efficiency. Energy Conserving II oils should provide the best fuel efficiency, followed by Energy Conserving oils, and finally, followed by the lease fuel efficient oils, ones that don't contain either designation.

SYNTHETIC OILS
Compared with conventional motor oils, the synthetics involve different chemical processes and ingredient percentages. However, the base stock still comes from crude petroleum, and the additives are virtually the same as those commonly used.
From a performance standpoint, they work well, especially for engine start-up at very low temperatures. But, based on all the information, synthetic oils can not be left in an engine any longer than conventional oils (extended use is the principal claim of synthetics). They can be used, but must have the right service category, viscosity grade and fuel efficiency designation for the vehicle. The oil must be changed as specified in your owner's manual.
A second category (beginning with the letter "C") also exists. These oils (CC, CD, CE, etc.) are intended for diesels, like in trucks and bulldozers, and by themselves are not suitable for gasoline engines.

VISCOSITY
Viscosity is resistance to flow. Thick oils have a high resistance and, therefore, a high viscosity. Thin oils have a low resistance and, by the same token, a low viscosity.
A Society of Automotive Engineers (SAE) number that appears in the center of the API logo. The SAE number defines the viscosity grading of oils, both single-grade and multigrade
Single-Grade Oils - "Single-grade" refers to oils whose labeled viscosity applies at one temperature only. In general, single-grade oils indicate their viscosity by a single number, for example SAE 10W or SAE30. The higher the number, the thicker the oil. The W (for winter) tells you that the labeled viscosity applies at a sub-freezing temperature. Otherwise it applies at 212°F.
Most non-W oils thicken when the temperature drops, which makes cold-engine starting very difficult or downright impossible. Many W oils also thin out too much in warmer weather, which hurts their ability to lubricate and seal.