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 multi-grade
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.