I will explain in the easiest way possible..Api viscosity ratings,,you can consider them as oil speed of flow.
When you see a W on a viscosity rating it means that this oil viscosity has been tested at a Colder temperature.The numbers without the W(i.e after W) are all tested at 210 degress F or 100 degrees C which is considered an approximation of engine operating temperature
For example, a 5W-30 motor oil performs like a SAE 5 motor oil would perform at the cold temperature specified, but still has the SAE 30 viscosity at 210 F (100 C) which is engine operating temperature. This allows the engine to get quick oil flow when it is started cold verses dry running until lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number is obvious. The quicker the oil flows cold, the less dry running. Less dry running means much less engine wear..
Will behave exactly the same in cold start..Same viscosity/same speed of flow.. but not at optimum running temperature(when the engine is hot)
there viscosities when engine is hot will be
30(less viscous/less thick but faster flow rate)
40(More viscous/thick than a 20w30 and less flow rate)
50(Most viscous/thickest than both 20w30 and 20w40 but lowest flow rate at optimum temperature)
These oils are called multi-grade because their viscosity/thickness varies with temperature.