In 1971, shortly after Mobil 1 was introduced, the exact same urban myths about causing oil leaks and not being able to change once you switched was bandied about.
It was nonsense then...
well you know.
For My K6 I use a non synthetic 20w 50 because of the heat here. In the winter I use 10w 40.
Because I change the oil often the additional life provided by synthetics is not required, so I do not use it.
I do use a synthetic blend in my Vulcan because I let it go 4k in between changes.