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 subfreezing 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.