I switched from Yamalube to Amsoil "HP Injector" about a year and a half ago. I just changed my plugs this weekend after about a year and half as well, and the plugs didn't have a bit of carbon on them.
When I fished the CBBT this past winter, I left my motor running in idle 4-6 hours at a time, for a few days, and it never stalled or failed a plug.
I am a believer that 100% pure synthetic oils "And Not Just Amsoil", are superior then standard patroleum stock TCW-3 2 cycle oils.
I am interested in getting around to inspecting the o2 sensor, and see what kind of shape it will be in as well.
P.S.
I have been ordering my oil directly from Amsoil from their online store, and they ship it to my house within a few days.