Dogma
Forum Sage
" ...Synthetic base oils give enhanced performance because they are manufactured using advanced chemical processes, so their molecular structure and hence, their properties, can be closely controlled. For example, fully synthetic oils like Shell Helix Ultra are designed to flow more easily at start-up temperatures.."
http://www.shell.com/global/product...ricants/cars/how-oil-works/synthetic-oil.html
The viscosity rating describes two points on a temperature vs viscosity curve. For similar chemistry oils, this curve should have the same shape. They've monkeyed with the chemistry enough to alter the shape of the curve to favor lower viscosity at temperatures below that where the first number of the rating is specified.
Still, this is not a unique feature of synthetics. Dino oil formulations are subject to similar manipulation, though maybe not to the same degree. So, if your gaskets are marginal, what you have to look out for is oils that claim to lower viscosity at low temperatures (Probably lower than you'd ride. Well, maybe not in Wisconsin.), where the higher viscosity will result in higher pressures in the passages. Also, if you think your gaskets are marginal, avoid higher RPM than you've used before, because that also results in higher oil pressure.
Honestly, if I'm worried that a bike may start leaking from such a small disturbance, I'd be looking at new gaskets. Worrying about mechanical issues is no way to enjoy a ride. Especially when the only seals exposed to full pressure are the oil gallery, oil pressure sensor o-ring, base gasket [o-rings], and head gasket. All the other seals are essentially exposed to just crankcase pressures from the pistons moving and maybe blow-by, or hydrostatic pressure in the case of the clutch and generator covers and a few shaft seals.
To my mind, the real reason to avoid synthetics in an old engine is that a tired old air-cooled engine is likely to be consuming oil, so you'll be burning, blowing or leaking more expensive oil.