synthetics have a more uniform molecular size, so the smaller ones don't volatilize or leak out and the big ones don't form a heavy deposit. that's it; not slipperier, no magic. just uniform size, which improves their viscosity retention (synths don't get as thick when cold, because they don't have the big molecules. That also explains the generally cooler running), and breakdown resistance (uniform molecules means no strays to crack easier when hot or under pressure). Being higher priced they generally have a good additive package, but that's not necessarily linked to their synthetic-ness.
Given their superior film strength and breakdown resistance, and flatter viscosity curve, why wouldn't you want synthetics in everything, all the time? Ideal for breakin, in fact, when you most want these virtues. I have consistently used synthetics in everything (including wet clutches) from new and never had a problem. Overkill on an Enfield, but what the heck, it only costs a few bucks (Shell Rotella T 5w40 synthetic is about $15/gallon). Even HD has finally given up the "bearing skate" myth and peddles their own synthetic, suitable for all holes on an HD (leaving room for some bad jokes here...)