So you could run 20:1 fuel mixture, but the plug wouldn't foul? Oil does not burn as gasoline does, and if there is too much oil in a fuel mixture, the plug WILL foul.
 Carb jetting can also be a contributing factor but only if the proper fuel mixture is used (which seems to be presented with conflicting evidence on every forum.)

Blanket statements are seldom correct, and I prefer to address each issue with an accurate diagnosis.