Some other advantage of synthetic oils for air cooled engines:
-They have a very linear viscosity transition across temperatures. Dino oils are thicker when cold, thinner and breaking down,as mentioned, when hot.
-You can use a thinner oil weight. One of the reasons for heavy weight dino oils is the higher shear strength. Since synthectics don't fail as easily on shear you can use a thinner oil. Thinner oil is a GOOD THING! To a point
The thinner the oil is the faster and more completely it gets to all the engine parts. You didn't have this option with dino because of shear failure but you have it with synthetics. You don't need to go thinner than manufacturer recommended but if you have the choice between two, go thinner.
Also, after much reading I've determined there is some benefit to moto oils if you have a unit engine with the same oil in the engine, clutch, and gearbox. Gears break down oil. Auto oils are legally limited in the amount of compounds they can put in to prevent this. Older oils had more but these are bad for the env. Moto specific oils have more of these compounds.
All this said, if you're not racing or riding really, really hard and you check and change your oil and filter regularly with good quality oil, which one you choose, brand, dino, synthetic, doesn't really seem to matter all that much. How many people do you know who have lost an engine to oil failure? I have almost 200,000 miles on my Civic. It gets name brand oil but nothing special. It has been to Jiffy Lube amnyt times in it's life. Still runs great and has great compression.
Scott