Most people are familiar with oil's Viscosity Rating—10W40, for example. However, very few know that the "W" refers to "winter," not "weight." And most of us have no idea what the weight-rating numbers mean other than that the vehicle's manufacturer specifies a particular viscosity.

Low-viscosity oils flow better than high-viscosity ones—the lighter-weight fluid is easier to pump and therefore circulates faster through the engine's various galleries. Low-viscosity oils also maintain a lower oil pressure, but the oil pump delivers a greater volume through the galleries than it would with thicker (higher-viscosity) oils. Heavier oils also tend to operate at higher temperatures because the oil pump has to work harder to force the lubricant through the system. Oil does not compress readily, so the added pressure increases the temperature. In the end, high-viscosity oils maintain a higher oil pressure, but the pump delivers a smaller volume of oil.
If a biker doesn't like 10W40, then use 20W50. It can do everything a 10W40 can do except pass the sub-zero viscosity test at -20 degrees C.

Your right about that and the different oil thinkness, but the W stands for warmth (heat) because the viscosity of the oil depends on the temperature of the oil.

If you have a ducati with a lot of milage the engine has more tollerance on the moving parts in the engine.
When you use a oil wich is thin when you do a coldstart the oil goes to fast and you will see a low oilpressure. This also explains the gearbox noise of the engine.
When the engine gets warmed up the tollerance will slightly dissapear untill it reached a minimum left - this has to be otherwise your engine will breakdown.
When you have a engine with a lot of tollerance because of the milage its better to use a oil wich remains its thickness even when its warmed up.
Conclusion is that synthetic oil is only good for engines with low milage - when you have a lot milage on the engine use a oil like 15W50 mostly its mineral because there is no full synthetic for that value or its a half synthetic.