What is the difference between 20w40 motorcycle oil and 20w50?

Basically the difference between the two is the ability for the oil to flow through a viscometer (scientific measuring device) at a temperature of 210 Degrees Fahrenheit in a predetermined amount of time in seconds.

Below you will see that an oil that is a 40 grade will flow through a viscometer at 210 Degrees Fahrenheit faster than 12.5 seconds but slower than 16.3 seconds. The same rule would apply for a 50 SAE Grade oil. See the chart below.

4012.5<16.35016.3<21.9

Now if you are trying to determine which oil would be best for your bike here is what I would recommend. If it is factory untouched bike I would recommend going with what the manufacturer calls for. If the bike has been modified extra horsepower, higher rpms, etc.. then you may want to consider an oil with more shear strength. In other Words if the bike called for 20W40 and has been modified you may want to move up to a 20W50. One thing to keep in mind a 20W40 oil will be more fuel efficient than a 20W50 oil. Something that when protecting expensive performance equipment is not usually considered but I thought to bring it up. Fuel economy or a new engine! Be sure that whatever oil you purchase has the ability to retain the viscosity that it is rated for. Many oils will break down (shear) and lose viscosity. I.E. a 20W50 oil could easily break down to a 20W40 Oil Or Even a 30 SAE Grade Oil which would not do you any good.

Hope this helps :0)