Well, I've seen it, and with name brand oils too. On motorcycles, ultralight airplanes, yard equipment, and model airplanes. Carboned up rings, stuck rings, carboned up pistons, exhaust ports. I recently took a friends Super Tiger .90 that had been running on gas apart, and I couldn't believe how carboned up it was. And it only had a few gallons through it, running just rich enough to hear an occasional burble....

In the study in the attached .pdf file, the engines were adjusted to proper mixture/egt temps, and look at the carbon....

AV8TOR