Seems like the OP and some others are running Amsoil and some other oils at 100:1 or 80:1, when they had previously been running a 50:1 mix. Amsoil Saber is considered very good oil (ask Richard Martin, for example).
I'm wondering if the problem is not the Amsoil, but changing the mix ratio from 50:1 to 100:1 or 80:1. Haven't heard anyone mentioning retuning the carb after switching to a different ratio.
It is my understanding that you need to adjust the carb after changing the mix ratio. Can't remember if less oil makes it run richer or leaner.