Oil isn't what fouls the plug up, it's the carbon residue from improper fuel combustion.
Remember that carbon is a type of conductor (they use it for brushes on electric motors), and you're essentially coating the porcelain insulator with it.
Build up that carbon residue enough and the spark will short out at the base of the insulator instead of jumping the gap...hence a fouled plug.
Burning oil doesn't create any more carbon residue than fuel does, unless there's no more oxygen to burn (fuel mixture too rich).
If jetted correctly and running, the spark plug will be hot enough to burn any oil trying to short the plug out regardless of oil ratio.
The practical limit to oil mix ratio is probably like 5:1 to 10:1, I dunno, it all depends on how long you run it at full throttle.
That kind of oil ratio isn't good for cruising around with your grandmother.