I use Mobil 1 in all my engines, even the lawnmower. Air cooled engines can use every advantage you can give them to keep their cool. The synthetic oil just does not break down....even after extended use, it's slick as the day you poured it in.
The problem with using synthetic oils in small enigne is that absent good filtration you cannot benefit from the longer life because oil still gets dirty. Lubricity and thermal resistance are good things but remember that oil also acts to suspend combustion products to prevent scoring of the cylinders that they cool and lubircate.
Leaving any oil in for longer than a year will dramatically increase the probability of damage from dirt not viscosity breakdown or excessive heat.
In the correct application synthetic oils are tremendous. In my TDI Jetta I change the filter every 6000 miles and the oil every 24000 miles. The benefit to me is much lower net cost of the oil change and my fuel economy benefits from the improved lubricity to the tune of 53 MPG at Interstate speeds.