The meaning of flowViscosity is a measure of the oil's ability to flow at a given temperature, which indicates how thick the oil film will be on metal parts. Obviously it flows more slowly and forms a thicker film when it's cold; a thinner film is formed when it's hot. Because automotive engines operate over a very wide temperature range, they use multi-viscosity oil that doesn't get quite as thin when hot or quite as thick when it is cold.The Society of Automotive Engineers (SAE) is the keeper of viscosity specifications. When SAE 10W-30 oil is cold, it will flow like 10-weight oil does in winter temperatures (thus, the "W"), and like 30-weight oil when it's hot. This doesn't mean that multigrade oil gets thicker as it gets hotter; it means the oil flows the same as straight 30-weight oil does when it gets hot. multi-viscosity oil is created by adding polymers to narrow the change in flow-rate over wide temperature ranges.In the ongoing effort to improve fuel economy, car manufacturers are designing their engines to use lighter oil, typically 5W-20 or 5W-30. Some of the hybrid engines are using 0W-20 oil. In any engine, oil has several functions, which include:* Lubricating moving parts.* Carrying heat away from engine parts.* Acting as hydraulic fluid for lifters, tensioners and actuators.* Sealing the rings to the pistons and cylinder walls.Today's engines are tighter and built with greater precision, so low-viscosity oil is quite adequate for these tasks. It improves fuel economy because it's easier to pump, which reduces engine power needed for the oil pump (parasitic loss) and creates less drag on rotating parts (windage loss). Once the engine is warmed up, using oil that's one or two grades thicker usually doesn't cause any problems other than reduced fuel economy. However, when it's cold, thicker oil flows slower, which can cause damaging wear during cold-start and, under certain conditions, can also prevent hydraulic lifters and tensioners from bleeding down as designed