Have been using synthetic oil, mostly Castrol 10 - 50 wt ( I think ) especially on the older bikes.
My '48 Pan, '46 Knuckle, and various HD's from 1914 to 1931 seem to like it.
Have even noticed a couple of cases where the bike seemed to pick up a bit more power, the longer they were run.
Synthetic also burns cleaner, cutting down on plug fouling and smoke from the older bikes.
The most recent example was a 1929 Harley 350 cc single, ran yesterday.
It went from clouds of smoke to barely any smoke since draining the crankcase and putting a quart of synthetic in the oil tank, which was also drained.
Picture, below, of bike going up Palomar Mountain yesterday.