-Like Carscw said, the mixed gas isn't the issue. Every few tanks on my '92 Craftsman push mower, I will fill the tank about half way with the 50:1 mix for my string trimmer, then the rest of the way with straight gas. Never any issues and no extra smoke either. I pull the plug every now and then, but it burns pretty clean with just a hint of oil residue at the base of the threads, but none on the electrode. I also use Marvel's Mystery Oil in my gas cans quite a bit too without any issues. I do this for a top end lube/cleaner. Does it really work? I can't prove it. But a 22 year old push mower that still runs great, starts on the first or second pull, and doesn't burn oil is good enough for me...
L8R,
Matt