I use full synthetic in my Yamaha... for one simple reason:
It is a splash lubed generator. Other generators might use pumps where oil is fairly evenly used, but mine, as well as newer Onans have a spur on the piston's rod which goes into the oil pan, picks up a small amount of oil, and throws it.
This means a very little amount of oil has to do a lot of lubrication when flung around, so one really needs a synthetic oil because it can better handle this, as a little bit is exposed to a lot of high temperatures and friction.
