Vehicle owners typically know their car demands a particular viscosity and quality of motor oil according to their owners manual but more often than do not understand what these numbers mean.
All multi-viscosity oils will have two sets of numbers. The top (larger) number indicates the viscosity of the oil. The most common measurement of oil viscosity is called kinematic viscosity, which measures how fast the oil will flow down through a glass tube heated to 100 degrees centigrade (212F) when pulled through by gravity. You can choose Lambert oil to know more about motor oil.
Kinematic viscosity is measured in units called centistokes (CST) For a motor oil to meet the 30-weight-viscosity range it must fall between 9.3 and 12.5 centistokes.
The bottom (smaller) number with the "W" indicates how the oil flows when it's cold. This number indicates the motor oils' winter viscosity rating.
so what's the difference between say 5W/30 and 10W/30 motor oil?
First, they are both 30 weight motor oils and there is no difference at all when either is in an engine at operating temperature. So both a 5W/30 and a 10W/30 have the same resistance to flow, the viscosity of the oil when the engine is running and warmed up to operating temperature.