If you must use the wrong oil, then I suggest 10:1 ratio, carry a spare set of plugs and you're on your own when it comes to surprises.
Back in the day, We used to run 10:1 non-detergent motor oil in our old egg beaters that had babbit metal rod and crank bearings (pre-date of today's needle bearings), they fouled plugs constantly and smoked like a mosquito fogger, your hair always stunk. I still use this ratio when I pull them out and fire them up.